If you're still enjoying Sims 3, then I would say that there's no reason to buy it right now. First and foremost because it doesn't really reinvent the wheel--it's still a Sims game, just one that's wrapped in prettier packaging, but also because it goes on sale frequently enough--often at 66% instead of 50%.

In the end, though, it's still a Sims game and there are enough mods out there to make it the kind of game you want to play, with the exception of things like toddlers and the open world. Expansion packs have been a little more miss than hit, IMO, but that's largely because I'm not interested in things like retail or following my sims to work or an enhanced socialization system. Other people are enjoying them. Content releases have also become a little slow, and I've seen people mention that they keep losing interest, so that's something to keep in mind. I myself only managed to play for a week or so before everything seemed repetitive or needlessly linear and quest-like (do x and y every day to advance your sims more quickly in their careers, fulfill x and y to have a successful date). And I barely owned any Sims 3 expansion packs but still managed to pour a fuckton of hours into the game, so it wasn't just because I was only playing the Sims 4 vanilla.

tl;dr, If you're bored with Sims 3 but still want to play a sims game, go for it. It'll still give you the Sims experience. Otherwise? Naw, have fun with what you're doing.