Just started running 3.5 again after taking a break and running/playing occasional non-fantasy rpgs. The game I run is heavily houseruled and uses significant influence from PF and other sources, but the 3.5 books are still my fundamental references.
I use it because it's the best system on the market and I'm invested in it; to even consider changing I'd have to see a game that offered flatter, more balanced math, more flexible character creation, and maintained or enhanced the fantasy "feel". PF is kind of a sideways step, 2e is a step or two back mechanically, 4e is about ten steps back on every level, and 5e is looking to be a few steps backward in my view. So again, I play 3.5 not because I have to or because I'm intellectually conservative, but because it's the best thing out there.