My DSLR is aging out...they've changed lens mounts (manufacturers seem to do that just long enough to blow the cost benefit of keeping lenses long term...I use my phone for 98% of my photos. The last one I actually printed out (a panorama) came from my iPhone XS Max.
I just can't justify a $1200 camera purchase to replace my Sony A57...and I'm a little sad about it.
It must have been the move to mirrorless that forced a mount change. Even Nikon abandoned their F mount line (which started in 1959) and introduced Z mount for their mirrorless.
Also given how often Sony is churning out new cameras these days, going secondhand will save you serious dough.
I think you're right on all accounts. The adapter to let the old lenses work on the new bodies is the thickness of the pentaprism on the old camera. It's designed to keep the geometry of the stuff that got removed when they went mirrorless.
A lightly used A-mount (like an A77) is an intriguing thought...if it had a kit lens, then my kids could keep using the a57 til it failed completely.
I just can't justify a $1200 camera purchase to replace my Sony A57...and I'm a little sad about it.