For me I found the new Reddit desktop site completely unusable. I hated everything about it. On desktop I used old.reddit with RES.
On mobile their 1st party app was similarly shitty like their new desktop ui. The 3rd party apps were much better but I didn't necessarily prefer then over the old+RES desktop website.
Main reason I also used mobile apps was that I could browse Reddit while taking a shit.
Yeah that's a great point. I think it would be hard to fully lock other clients out, but you could have an early internet style situation where you had some websites not supporting all browsers.