Just this week, I uninstalled all my desktop mail clients: no more Thunderbird, MailMate, or eM Client.
Fastmail's web app feels faster, uses fewer resources, and has the same UI on all of my computers.
Instead of redirecting all other accounts into Fastmail, it has a simple UI to setup fetching-from and sending-as from other mail providers, and with using my own domain, it does not feel like a lock-in.