I use Fastmail because of the excellent mail service, customer support, sieve rules to keep my inbox manageable, and because of the little details like WebDAV that I have unexpectedly found useful for things like syncing browser bookmarks. I pay for Fastmail because it has been excellent, not because I’m captive.
This app signals to me that there may be a change coming in our relationship. One in which you may start to neglect IMAP or make it harder for me to choose my own mail client. It tells me that Fastmail might be pivoting into “growth” and treating me like a “user” rather than a paying customer. It won’t surprise me, and I can’t really say I’d be mad. Most companies get to that point. There’s probably a lot more money to be made from vendor lock-in than happy, $50 per year customers. As for me and mine, I will switch to the next small-to-medium-sized mail provider the second I get a sense that this is the direction.