And as a non American, I support the above. My email archive goes back to the days prior to Gmail, prior to my current email address, infact even prior to using Thunderbird (it's been in Eudora, Netscape Communicator, Mozilla Suite, and Thunderbird variously over the years).
I keep the last month or so's email live on my mail server and read it with IMAP. From Thunderbird. On a half dozen different machines. Windows and Linux. All Thunderbird.
Every month or so I use POP3 to pull all the email server-side down to my archive installation of Thunderbird on my home server.
I refer to my archive about every month or two at minimum, and have already found value in being able to pull transactional email notifications from 2 years ago out of an archived folder, to help rebuild a mailing list that was hosted in the cloud (but where said cloud service provider decided to be nasty and delete an entire VM, plus backups, simply because they could, and not because it was reasonable).
My email archive is mine, it's on hardware I control, backed up by my own backup regime, sitting in property I control and subject to local jursidiction. My live mail platform is one I personally administer, that I can read from the several computers I use week-to-week using exactly the same software (Thunderbird).
I'm glad to hear Thunderbird will still have 'some' attention, though I hope the writing isn't on the wall. We need Thunderbird. My entire corporate office uses Thunderbird + Lightning (Mac and Linux clients) to talk to our POP/IMAP platform and soon, to talk to Zimbra. Zimbra might be a powerful web based app but it's still nice to be able to carry out work when you're disconnected!