Good question.
Thinking back to the early days of photography, the Leica rangefinder was a beautiful simple machine for taking pictures. Its drawback was parallax, the view through the viewfinder not exactly the same as through the lens. And although it had interchangeable lenses, alternative focal length lenses presented problems in the viewfinder. Various solutions such as guidelines in the viewfinder, and auxiliary cold shoe mounted viewfinders, were tried but you still had to use the main viewfinder to focus with the rangefinder.
The single lens reflex was a solution to these problems but it presented its own issues and added to the cost and complexity of the camera. I'm sure the clever folks at Nikon, Canon et al are thinking that now that we CAN dispense with the mirror box, why wouldn't we? It creates so many obstacles to camera design that dispensing with it frees up the designers to innovate in many ways, not least new compact designs of lens.
You can now put the rear element of a lens much closer to the sensor, facilitating new innovative designs of lenses, particularly wide angle lenses. It's a little surprising to many of us that the new lenses coming on the market for mirrorless cameras are mostly even bigger than their SLR ancestors but we can live in hope.
Looking through an electronic viewfinder is an odd experience for an SLR user, but it has advantages - you can see a nice bright image even in the dimmest of light when you'd hardly be able to see through an SLR viewfinder at all. And the camera can focus at smaller apertures, so crazy long lenses built with stacked teleconverters will still focus.
Everything changes, and I think the SLR has had its day. When autofocus was invented I thought it wouldn't catch on. I was wrong. Digital cameras took over from film and I think mirrorless will take over from DSLR.
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R6mkII, various lenses, speedlites. Also legacy Canons going back to T90 and even A1.
