While I really don't care about this new app since I'm very happy with their IMAP/JMAP support, there is a fact that you don't know at all. You have no visibility on the number of users asking for a desktop client. I adore Fastmail (subscriber for 14 years now and counting) and they have a history of shipping only what users want. So perhaps you are upset, but I bet that this addresses a need from their user base.