People can't host their own software. The handful of people who can host software won't install security updates or deal with backups. It's a drag. It makes no sense for millions of people to individually manage servers and server updates when it can be done centrally for a fraction of the cost. That's why SaaS won, ultimately.
I don't believe for a second 37signals is going to make desktop apps that you just install to your machine. These will be dockerized web apps with many fragile dependencies. This is a step backwards, frankly. If you want to make buy-once-zero-maintenance software the client software should automatically join a fully distributed peer-to-peer network. The alternative is for 37signals to offer the digital service part at cost.