Sometimes it is difficult to try and understand why Canon does some of the things they do. My belief is the M series was just a stop gap for mirrorless when Canon had none and the others did. It is basically a Powershot with interchangeable lenses.
Why they never come out with a 150-600mm (or simular FL super zoom) when they were so popular remains another mystery to me. Of all the folks I know that shoot more than just an occasional snapshot have one version of the 150-600mm.
Another is why make these super slow zooms for the R series. If I had an inclination to buy an R camera that alone would be a deal breaker. Nada, never.
But what'da I know? 
@ebiggs1 wrote:Sometimes it is difficult to try and understand why Canon does some of the things they do. My belief is the M series was just a stop gap for mirrorless when Canon had none and the others did. It is basically a Powershot with interchangeable lenses.
Why they never come out with a 150-600mm (or simular FL super zoom) when they were so popular remains another mystery to me. Of all the folks I know that shoot more than just an occasional snapshot have one version of the 150-600mm.
Another is why make these super slow zooms for the R series. If I had an inclination to buy an R camera that alone would be a deal breaker. Nada, never.
But what'da I know?
I think that the M-series suffered from a terrible numbering system - the models seemed to be assigned numbers randomly - I admit probably only frustrating people like me who were aware of the numbering system for the EOS series and were looking for some associated logic. The target audiance seems to have been travellers, backpackers and social media v-loggers considering the majority of the designs emphasized compact size (at the expense of EV's generally), limited FL ranges, decent video specs. The lack of long lenses made sense considering the majority of the models didn't have EV's.
For those of who want a small, compact photographer's camera with an EV built-in (as opposed to a clunky and expensive add-on) there was only the M-5 and M-50. I consider the M-5 to be the photographer's camera in the range as it has the EV dial on the top and is not as well spec'd for video as the M-50. Admittedly the M-50 came out much later and it has been a massive hit with v-loggers for some time. As many have commented, the range of lenses is neither huge, nor fast (generally), but for those outputting to social media or digital devices the quality is OK. The 22mm, 33mm and 11-22 and 55-200 have got a lot of cudoes for their quality and value. The rest do OK IMHO. Again, to me, it's all about the output...
For myself, I got my M-5 units for when I travel or tramp but don't want either bulk or weight to burden me. Like you, I would like to have seen something more at the long end - like the rumoured 100-300mm. If they come out with an M-7 (is that the M version of the EOS 7DIII??
) with the specs as rumoured, then I think it would be a useful body. I have no use for the M-50II as, again, it seems to be targeted at videographers.
A super zoom like the 150-600 is going to be big, or involve a lot of compromises to fit comfortably on an M body, and personally I would prefer to have that attached to a DSLR for balance alone. As to the two super tele lenses to which you refer: I am not likely to be in the market for them as I have a 150-600, plus a 100-400 with 1.4 tele and on a crop sensor body they take me close to an equivalent FoV of close to 1m and give me results I am happy with. That's enough for me! 
For me, the big question is whether I expand to the RF mount. In a year or so, when the dust and prices have settled, I would consider an R6 with the RF24-105 f/4 and RF100-500. Those two would get me all I am likely to need, especially considering I have this huge inventory of DSLR gear.
