Gmail with html only would not be a nice experience. Modern gmail is really bloated but it's actually one of the few web apps I have no problems with.
Gmail used to have an html-only version if I remember rightly. Perhaps still does. It was faster and perfectly usable.
Hey the email services has just proved you could offer better than Gmail experience with HTML + small dose of JS. Another example being the new FE on Github.
At the end of the day it isn't really the tech that is the problem. Is how people use the tech. And for thousands of different reasons keeping it simple has always provided better experience evaluated on the whole.
Not really. I used the HTML version for well over a decade and it was absolutely fine. I guess if you need fancy animations, maybe that doesn't suit you, but I came through Pine and Eudora and Gmail HTML was a million times better than both of those and entirely sufficient for a media that dates back about 50 years.