I don't want flexibility and agility (i.e. a different UI every three months), and I certainly don't want to be offered "products and services" when reading my email. I have been running Thunderbird on Linux and Windows and wouldn't know what to do without it. Works fine with my employer's Office365 server (whose abysmal UI I occasionally have to touch when somebody sends one of those Calendar invites without mentioning in plain text where and when).
As for focusing on the browser: I don't see the fuss. I generally use Firefox, but don't see any noticeable difference when occasionally using Chrome or Edge. As long as there is a way to switch off autoplay and block ads, they are all the same.