People have been asking for the ability to create playlists/collections, the information panels you get for each game, etc., for years, and the only major negative (if you can even call it that) feedback that they got was some UI stuff that some people want to be able to change, like the ability to disable the "what's new" section and stuff like that.
there will always be people that don't like change, there were a couple of those during the beta as well but nothing major.
My best advice to you, get to know the new UI and the functions,
Also, anyone that just leaves a platform as soon as a change they don't like or agree with is made wouldn't exactly be a reliable or useful beta tester.
I'll address those points because they're problematic:
1/ There's no reason to link features to appearance. You can add new features to an old UI or change an UI without adding features. Keeping a list mode or a small mode doesn't preclude adding features or another mode.
2/ Not liking having my Steam library changed to a smartphone screen has nothing to do with not liking change. That's why I didn't use Windows 8 (note that Microsoft backtracked quite a bit on that with 8.1 and 10), and that's why I've been consistently using Galaxy 2 since the release of the new Steam library update.
There's a density of information in list mode that can't be matched when 80% of the screen is full of pictures. Losing that density of information is a problem. Denying their customers the choice is a problem.
3/ I've poked and prodded the new UI. The new features are good, the new Grid Mode in itself is trash. Criticism of the latter isn't rejection of the former. I make informed choices, Steam decided to remove that choice from me.
4/ Steam isn't a beta test. The new UI is now part of the commercial release of the Steam client. People leaving are not unreliable beta testers, but customers making a choice. Do not take one for the other.