Do you see a regular user researching, downloading and setting up a third party email client or is it more likely they click the download button on Fastmail and log in with their account?
That’s the beauty of open standards, everyone can choose their favorite tool for the job depending on their preferences and skill levels.
No, I see them using the web interface actually, just like they've done so up until now.
I don't think for instance this was keeping a lot of people from switching to Fastmail from let's say Gmail, which also doesn't offer a desktop client.
If Fastmail has an adoption problem it's still from people not wanting to pay for their email, a desktop app is not going to change that.
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.
That’s the beauty of open standards, everyone can choose their favorite tool for the job depending on their preferences and skill levels.