Desktop MUAs won't suddenly become popular now. A generation of users has been brought up on webmail, and the idea of using a separate program for email on the desktop isn't familiar to them.
The person in question is someone who became "computer literate" relatively recently (at least compared to me, I'm 37 and always been tinkering since I was a kid) and has become accustomed to most on-line services while using smartphones and tablets, not a personal computer.
That person is not alone. To be honest, I think most people under 20 and most people living in developing countries went down a similar path (mobile first) and have a similar understanding of messaging platform.