the people still using a fat client are likely doing it because they don't like change

edit: I say this MYSELF as a thunderbird user!

I don't want an "new experience" every 9 months, and having to explain it to my parents

Or people that simply have more than one email account?

And I use a fat client because I like having all of my email addresses aggregate to one place, and I like it when that software gives me a modern look and feel ¯\_(ツ)_/¯