I “downgraded” from an R6 28-70 to a 1DX with a variety of lenses. It all comes down to what you do.
Yes the R6 is more advanced, IBIS, cleaner shadows, 2 more megapixels but in the real world, it doesn’t matter that much.
IBIS definitely was handy for video and low light photography but not a deal breaker and EVF is a love hate relationship.
I’ve always preferred a bigger body so 1DX felt like home. First thing I did was try to touch the screen lol, took a bit of getting use to but because of the button layout, I can shoot just as quick if not quicker.
Plus with Adobe’s new denoise ai, it’s insane how well it cleans up a 12,800 ISO shot. Not only cleans up noise, it brings back color.
If you primarily shoot photography, DSLRs didn’t magically become bad, mirrorless just took over and provides a lot of convenience, doesn’t mean the pictures they get are better than a DSLR.
The biggest upgrade to me isn’t from the bodies or AF, it’s lenses, that 28-70 had zero issues where as EF glass has chroma, fringing etc RF lenses have really improved over EF especially primes like 50 or 85 f1.2.
The main reason going back was that I felt super lazy with the R6. You hold the camera out and it does all the work for you. You don’t get that I worked for this photo feeling so I really started to lose the passion. Other reason is I want to upgrade to the 1dxiii by Christmas. That camera is the best of both. I can use it like a DSLR or use it like a mirrorless with live view. Has all the AF features from the R6/R5 too.
I think mirrorless is more beneficial for video especially with eye AF however for photographers, you still see a lot use DSLRs especially in sports as you have more control. Feels more raw.
Again all depends on what you shoot.