Edit: but it only works because of the space in a PC tower. It doesn't work for laptops, and I was sort of in the same boat as you. I couldn't imagine it would work for phones. Specifically that they are so space constrained.
Now modularity in PCs is the domain of gaming PCs, plus a few speciality niches. Gamers as a market could be seen as analogous to audiophiles: a bit cranky and hard to please.
DSLR bodies go through a release cycle that is not much longer than major phone models, these days.
One reason to put up with this crap is that you can save thousands of dollars buying nice old glass for your nice new camera. But even though the glass lasts forever, once you get 50MP sensors in cameras, old glass that was designed for the best resolution of 35mm film will limit the performance of a 50MP+ image sensor. Soon, photography will be mostly computational, and big old lenses will all be museum pieces.
I don't even want to think about whether I've kept the firmware on my Sony E-mount lenses up to date.