That's a lot of assumptions unless you know their internal numbers and customer feedback. I've talked to many people building products and often they themselves are surprised what's needed to get certain larger customers, even if they would never use certain features themselves.

I could very easily imagine that if a company wants to switch over their employees from another provider where the users had an app that had email / calendar /contacts all in one app that they would like to have that same setup again on Fastmail. In the end packaging their web app in a wrapper to satisfy that need of certain customers groups doesn't seem like something that should be very controversial.