What are the chances of a company owner agreeing with a strike? If they did then there wouldn’t be a strike

No one said that Thunderbird keeps copies of your email. But there are apps that do that and the OP does not want one of them.

No, I am saying that OP is looking for an alternative client, which also doesn’t upload their emails. For example, mailspring would be a bad recommendation because, as far as I know, it uploads your email to their server

They mean mail apps that keep a copy of your emails in their servers