If Thunderbird had a hosted web version, yes. Are you arguing that data portability and interoperability should never be possible if the receiving app is an online service?

Of course Thunderbird could send an automatic update that starts shipping your emails to Thunderbird's servers. You dont expect that, but only because you trust them.

unless I’m wrong thunderbird software has complete access to all your emails when you give thunderbird your email details. Of course, that does not imply that a specific thunderbird employee can read your emails, it is probably encrypted on that end but if they pull a switcheroo and download all your emails into an AWS instance, yes that might be possible (and probably wildly illegal too)