The problem is that making a business out of "less" is hard. Especially when dealing with the hellscape that is "enterprise sales". Both the marketing people and the customer love piling on additional features!

Two decades ago I was at a startup that moved from a Linux email solution to a Microsoft one. Why? For the CEO's blackberry and calendar integration.

There are loads of FOSS email solutions and providers. There are fewer "email+calendar" providers, and there are even fewer "email+calendar+desktop logon SSO" providers.