Actually BTRFS was a good choise and made sense when it was decided to be used. (and still does) BTRFS has good feature set that is used and needed on the device and is optimized to minimize FLASH wear.
The real problem is users cramming too much data on the device, so the more sensible choise would have been having the device with 64GB storage in the first place...
Totally different thing, Aegis was meant to be a barrier for users, whereas Jolla choise of FS is a blessing for you