I have kept playing the one save I started the game with and have no intentions for re-starting. I just keep building the world, roll with anything silly that happens, and then when the save blows up or something... well, I guess then I'll restart.
I do have started New Games but they were for something very specific e.g. a challenge which I don't want to mess with my "main world". Never have I truly re-started.
If things get boring, then I just focus on different sims, create new ones, or switch up my gameplay a bit. I love when something unexpected happens and the game kinda gives me "story direction". There's been several cases in which things "didn't go as planned" but this sort of dynamic storytelling is big part of the fun and I guess one of the reasons why I don't re-start.