I used to re-install windows to my family like once a year, but then I refused to doing it anymore but I offered to install ubuntu on their machines. It's being like 4 years so far and they hardly need any maintenance from me.

IMOE the main issue are software-suites

Cad and 3D: my dad is really used to windows, and learning the linux environment was an easy step for him. He could never do the switch to open source cad alternatives. We tried using wine, but for performance we ended up keeping windows as a 2nd bootable for him to draw.

Photoshop, and Corel: I installed ubuntu to a friend who works as graphical designer. She is currently using gimp, and inkscape but she had a hard time making the switch.

Games: my son really was the most resilient to the change. He played every blizzard game, and since there aren't (wasn't ?) linux support it was a big non-stop for him. Then he moved to minecraft, and that made him reconsider the switch. He's been using linux for about 2 years already -- no windows partition at all