Postbox unfortunately doesn't support Linux, besides that while I don't mind paying licences for great software, it doesn't look like Postbox is fully opensource [1].
I use gmail because of the labels (no, folders are inadequate), full-text search, and the fact that it runs in browser which I keep always open anyway. I wish there was a comparable open-source solution.
For me it is the fact that setting up a GUI client takes 10 minutes and learning to use it even less. If it was the same with mutt, I would switch in a heartbeat.
I tried gnus and mutt in the past, but gave up on them after a few days. I was spending too much time tweaking the config, trying to remember shortcuts or googling how to get it to work the way I wanted to. IIRC even displaying Greek with an ok font was a problem on gnus. That was 10+ years ago, could be that the documentation is better now and a good-enough setup is easier to achieve.
By way of contrast, people who use Outlook or Thunderbird feel comfortable in ten minutes, but they never make much progress.
If you deal with a lot of email, using the right tools is important. If email is not your primary communications method, there's no point in putting in the time to learn more advanced tools.
If you use a not-busted mail client (like mentioned above) you can actually participate in project mailinglists.
TUI clients are where it's at.
I wish. My mailbox is 80% attachments or embedded image nonsense.