So here's my current situation - I have an EOS 60D that has served me very well, and an assortment of lower-end EF-S glass (18-200, 10-18, 55-250mm) that have been fun to shoot with. I have some Canon accessories (580EX II, remote, yada yada). I think I've grown out of this setup now though, and am itching to upgrade.
I also have an EOS M100 and the 22mm pancake that I bought for a trip recently - a combo I've really enjoyed, but limited in many ways (no EVF, no hotshoe).
I do a wide variety of shooting - some landscape, some cityscape, some candid portraits, occasional events (amateur only, when they truly can't afford a pro). I dabble in video so it's a nice feature to have but not a primary motivator. I'm itching to upgrade as I'm finding the low light performance of the 60D inadequate - I had been toying with the idea of picking up the EF-S 18-55 f/2.8 for a while, but I don't think it's going to fully solve the problem and I don't think I want to spend anymore money on EF-S glass.
I'm looking at a few upgrade avenues:
Option A: I could dive into the EOS RP + RF 24-105mm kit - it's very tempting, especially at $2849 w/extra battery and converter. This would (a) give me the "step up" lens I've been looking for, (b) be a gateway to full frame, and (c) still allow me to use some of the "fun" EF-S lenses (10-18 and 55-250mm) - I understand there's a crop but I just don't use these enough to justify replacing them yet. What makes me hesitate is the limited 4K options... I'd really like to keep this body for 8-9 years, like my 60D, and the crop and frame rate limitations make me worry a bit.
Option B: I could pick up an EOS M50 body - I've been happy with the quality of the M100's images, but the M50 offers the EVF and controls to make it workable in more situations. It also scratches the 4K itch at a relatively low cost. The goal of picking up the M50 would be to last me 2-3 years until Canon (hopefully!) releases an RP update with better 4K options. It would not, however, open up any better lenses, and while I've considered picking up a used EF lens to use for it I'm hesitant when my long-term plan is to switch to RF eventually.
Option C: I can ditch Canon entirely - the Fuji X-T3 is quite tempting after borrowing a friends'... this is definitely the most costly option though, and there's no low-cost equivalent to the 10-18mm I've come to love in the Canon system.
I like the RF mount, because it offers an excellent upgrade path from the world of APS-C bodies and EF-S lenses, to the world of full frame bodies and EF lenses. Let me repeat that. I like the RF mount. At the same time, the RF mount casts a dark cloud over the M mount, because it is not compatible with RF mount lenses.
I not on the bandwagon with the EOS R bodies just yet. I just wish they had given the EOS R dual card slots, a faster frame rate, and a joystick, instead of the control pad. I could live without a joystick and dual card slots, but the fps is what gives me pause. Canon really should have given it dual SD UHS-II slots, IMHO.
As far as 4K video performance goes, every consumer camera body currently offering 4K video has some sort of compromise in how it is delivered. Anyone who is that serious in video goes the external recorder route, anyway. A clean, ncompressed HDMI output is more important that onboard 4K processing and recording, IMHO.
That is what I think of the EOS R and RP, and the RF mount lenses.
I have an M3. I use primarily one lens with it now, the EF-M 22mm f/2 USM.. Before the 22mm release, I would only use the EF-S 24mm and EF 40mm pancake lenses. I was not too thrilled by any of the EF-M lenses at the time. The M3 and EF-M 22 is an awesome combo for shooting in urban locations. I have no need to upgrade my M3 until it quits working.
I think upgrading a working camera body should mean a significant technological leap in performance, and the EOS RP has that over the 60D. The crop factor with using EF-S lenses on the adapter will be outweighed by the increased dynamic range and ISO performance over the 60D.
Like I said, i think the EOS R family is a good upgrade path from APS-C bodies. I just think the two cameras they have released were not quite what I would be looking for. I primarily shoot with a 6D and 6D2. I am no professional, just a guy who was ordered by the doctor to get out and walk. My cameras help me do that.
If you have been happy with the M100’s images, then do not replace it until it quits working. Ask yourself, do you need 4K video? You already said that video was not a priority for you. If you picked up an M50, then what happens to the M100? This is your call to make.
I think the images from the EOS RP and the RF 24-105mm L lens will probably make your head explode, to be honest. Seriously, I do not say that in jest. I think this EOS R family could be a bettter upgrade path than another investment in the EOS M family.
There is just so much uncertainty as to which direction Canon is headed. As of right now, Canon looks like they are throwing stuff at the wall just to see what sticks, IMHO. I suspect that the introduction of the long rumored replacement to the 7D2 may lift the veil, and offer up some clues as to the futures of the 4 incompatible lens mounts Canon has. Something has to give.
No doubt I agree with wadizzle. The RP piqued my interest when released. The following day however, I saw the specs, Battery section > Number of possible shots. (Wadizzle had pointed this out as well) > 270 best case scenario.
My 6D2... well over 1000 if I don't use live view.
I don't care how bad a$$ the RP is. I am quality over quantity for sure, but I feel the RP is hobbyist grade at best. Mirrorless... (someday) but for now, I'm waiting. "R" is first gen. But we all have different needs. If the RP fits yours, by all meant then.
I have three Canon 60Ds that I actually still use from time to time and personally - with the right lenses and these are not the ones you own - I find the units still produce great results. But I am not you and not photographing under your conditions.
I have the Canon EOS M5, along with a selection of EF-M lenses. Again personally I would go for the M5 over the M50, which is a somewhat downgraded unit as a stills camera, but perhaps is more of a compromise towards a videographer's needs. I note that you are apparently not THAT serious about videography. IMHO I think the EF-M mount will be around for some time as it represents to me Canon's approach to what was the compact market - very small bodies and lenses that are interchangeable - great for the traveller for example.
Like my esteemed colleagues I look to the new mirroless FF units as being the precursor to something much better to come down the line. In that respect I think both Nikon and Canon have "kept their powder dry" while they test the market and get some quality glass out there to support their bodies. In that respect I think Canon have been far more progressive and for me the excitement is in the potential in the mount and the glass rather than the current bodies. I would describe that market as being in its infancy.
So you do have some options...
1. Keep your investment in the M series and perhaps get the M5 (or if you are really that into video the M50), and some more EF-M glass - that depends heavily on what conditions you are photographing in. You say the 60D is not good enough in low light, but while the newer bodies might be more efficient in that respect, the EF-M lenses are not known for their speed. When they get the light I find them OK.
2. Consider waiting for a period for a body upgrade and instead get some good glass: for example the 70-200Lf4 MkII lens is a beauty and you could get a 24-70L lens to complement that - both will send more light to your sensor. The point is that you will be able to up your game with the 60D, yet these lenses will work with the EF-M (via the adaptor) and any future R series body (with an adaptor). If you go in either direction with a new body you are going to need to get replacement glass at some stage in any case (I know you have EF-S lenses and perhaps an adapter for the M mount), and while you wait a few months for the market to show its cards, you have not committed yourself, but you do have lenses that will perform well on either platform in the future.
3 If you really want to invest in the future and are prepared to commit to an upgrade path then go for the FF mirrorless deal you outlined. You won't (to my knowledge) be able to use the new lenses with your other bodies but you will have a platform to go into the future - but it won't be cheap...