I'm using a MacBook Pro from 2014, after putting in a new battery recently it still works great, and when I'm at my desk I plug it into a big monitor. Not as upgradeable as a desktop, but even assuming I can't sell the laptop when I upgrade it, it cost me less than $500 per year.
I've tried using multiple computers in the past (big desktop + MacBook Air, costs about the same in total as a 15" MacBook Pro that is much less powerful than the desktop) but I realized that the convenience of knowing that all of my files are on one computer is more important to me.