they have an amazing team, integration with 1 pass, tons of good things and chose a shortcut like every other major company because it generates revenue, short wins over strategy
Fastmail does, of course, probably consider its UI chops etc to be part of their value add. Just seems like if they also want to win over people who like native clients then "here's a bunch of shit that makes native clients also work very well with our offerings" is less of a lift while being more helpful overall. Maybe.