As much as I'd this to be true, sadly Google does not care. Easy example that I can think of: access google.com from chrome and Firefox (on Android). At least until a few months back, it still had that ancient UI on Firefox. Changing useragent, of course, made it the "modern" version.

Who will, or even can, bell the trillion dollar cat?

Edit- just tried with Firefox Focus, and sure enough, it's the "old" UI. Screenshot - https://i.imgur.com/dfRvBIW.png (With chrome it's the regular/modern one but I was logged in and couldn't screenshot in incognito)