Interesting. I switched to the new Gmail, and I actually can't remember what the old one looked like already. I'm sure Google will do just fine ignoring all these requests and their billion+ users will adapt.
My biggest issue (that users will find hard to ignore) is the lag. The new UI loads noticeably slower... And I don't feel like it's giving me anything useful to compensate for the lag.
The biggest problem in my opinion is not the new looks, but the degraded performance.
It seems after just opening GMail the website tries to cache every listed email in your inbox. After that is done, it becomes faster. My use case is not to leave GMail open in a tab all the time, so I have to suffer through this caching period every time GMail is opened now.
This, together with the recent Google-China upheaval, is the final straw for me. I've had a GMail account since the beginning, and am now looking into alternatives (ProtonMail, self-hosting).