The NYT published a very well-reported case (two cases, actually) of Google doing this more than two years ago. This is not a hypothetical -- this is already happening.
https://www.nytimes.com/2022/08/21/technology/google-surveil...
https://www.nytimes.com/2022/08/21/technology/google-surveil...
That's called "using an email client" to access email.
The way it originally worked, you know, and still does[1].
I use Vivaldi browser, which has a built-in mail client[2]. You can use Mozilla Thunderbird[3], or (gasp) Outlook.
All of them allow you to maintain a complete, offline, up-to-date copy of your mailbox, which you can export and back up if needs be.
[1] https://en.wikipedia.org/wiki/Internet_Message_Access_Protoc...
Apps like imap-backup (https://github.com/joeyates/imap-backup) allow retaining IMAP data.
In short, don't rely on TB to retain IMAP data, (like Dropbox) it is not a backup and IME will sync data losses, deleting local data if it is no longer accessible on the server.
Take care.
This is absolutely ridiculous of Thunderbird.
That said:
1. As you said, like Dropbox, it's a local mirror, not a backup. Someone gaining access to your email and mass-deleting everything would delete your local copy too. A an actual backup of Thunderbird mail files would accomplish a backup.
2. Did your ISP somehow allow Thunderbird to log into the mailbox and gave it empty content? Or did they delete content before killing IMAP sessions? If so, it's outright malicious. Otherwise, Thunderbird shouldn't have deleted anything.
3. Having an always-on desktop box with a mail client that gets all email once a week is a lazy way to decrease the chances of being affected by something like that. Having a script that copies mailbox data into another folder (named according to date) is, well, an actual backup.
It will dump your Gmail data into a mbox file, which is reasonably well supported.