I don't know why people would buy a camera because it worked well with other manufacturers' lenses. If they do, it is very naive of them to expect performance on par with the OEM bodies.
I have always been looking for a small, compact camera that can do most of what I do with my Canon bodies and deliver the same if not better IQ. Needless to say, the lenses too would need to be smaller. If such a camera does appear on the market and is also able to use other third party lenses (albeit not perfectly), it is simply the icing on an already nice cake.
To me, the A7 series from Sony did just that. Not perfect, for example it never made a good cup of coffee, but it did deliver on most of what it promised.
I tried the Leica M system before the Sony came along. Bought the M9 and later the M240 as well, along with Leica and Zeiss lenses. Total expense - $20K. The Sony A7RII with equivalent lenses - $8900. IQ, ease of use, AF, versatility (primes vs zooms), fps, expandability (third party equipment), high ISO performance, all so much better with the Sony. My own comparison of sharpness, color, contrast, distortion showed me that a lot of the legendary Leica quality is hype too. The only down side with the Sony being the larger size of the lenses. But, overall satisfaction way higher.
Everyone has their own needs and expectations. I am sure many here love their Leicas for various reasons. We all make mistakes, buying stuff that we later regret, I've done enough of that. However, sometimes it does work out.
Right now, in terms of IQ and size, I can honestly say my A7RII with the Batis 25 is just about the best combination of lens and camera I've ever owned. Period.