I've only ever restarted a save once, and that was with my first experience playing a legacy-style family (by my own rules). I ditched the first effort pretty early on because I discovered better ways to achieve what I wanted to do. I still have that save three years later (or rather, a series of "save as" versions of it), but I now rotate to other households as well as the original one, and I find that I need to rotate out to other saves pretty often to prevent myself burning out on that style of play.
As for most of my other saves, if a story isn't working well or if I take a disliking to a sim I made, I don't start over with them. I either incorporate elements of those partly developed tales or characters into other stories or get rid of them entirely. Occasionally I'll go back to an earlier story premise once enough time has passed to let me see it with fresh perspective, but more often I move on to something new. After all, there are infinite stories to be made, and even if I live a thousand years I'd barely scratch the surface, anyway. :lol: