Idk used to restart a lot when i get tired of playing my sims but this save is just endless fun so i haven't had that issue for 2+ years
next reasons to restart would be
1. save breaking beyond repair
(less likely now that I don't use mods but still possible)
2. Wanting to make completely different story for sims ive already played
(but i have decided to redirect that passion to playing more sims rather than constantly replaying same ones so i am still very much preoccupied with just playing every sim through once for a while)
3. wanting to rebuild world/lots ive already built
(but ive also decided to just work on completing all worlds all lots in one save now rather than using endless hours on that one favourite lot)