I don't really understand the outrage. The status quo is that companies didnt support it for years. So 99.9% won't notice any change.
But a mail CLIENT is a Web App not a static documents site. If Wikipedia would require JS I would kind of understand it from a technical point.
But big corporation tries to reduce cost by shutting down scarcely used old service happens monthly.
I think this is not the way.
It's like with dental care. The solution is not to don't care and think that someday technology will fix it. Instead you should brush regularly, don't eat much sugar and visit the dentist now and then. It's prevention.
We currently don't have any technology that will save us and time is running out. Why bet on a tech wonder that needs to be mass ready in no time?
Sorry but there are so much more things you have a way bigger impact on climate than website requires a little bit JavaScript.