I can't find any information on what happens if you stop paying for a Fastmail account. 1Password for example freezes your account in read-only mode. It's documented that Fastmail will re-use addresses for free trials and when a user requests to cancel [1]. It isn't clear what would happen if for some reason your card expired, they stopped accepting it [2], or your bank messed up and blocked the transaction [3].
To me, this introduces a new way to lose your account that isn't there with a free email service like Gmail.
[1] https://www.emaildiscussions.com/showthread.php?p=622760
[2] https://news.ycombinator.com/item?id=29988359
[3] https://www.reddit.com/r/personalfinance/comments/d1okxu/cha...