Almost everyone I know has converted over to Fairemail. For the following reasons:
1. One time purchase (no subscription necessary)
Yes, important. I also know users who use preinstalled mail apps, K-9 (Thunderbird) or Nine Mail & Calendar
There is much development on K-9.
By the way, a year ago Nine also tried a subscription model and failed. That was predictable. AquaMail will do the same.
The consequence for Nine was to rely more on the purchase version after all.
2. Regular updates and bug fixes (Aqua mail has been getting fewer and fewer updates and more and more bugs)
AquaMail was already a very sophisticated and reliable app in Kostya's time.
For me personally, it is not necessary to constantly receive new updates. Security updates are important, of course.