SimGuruLyndsay confirms The Sims 4 has no planned End-Date.

This does bode well for a personality overhaul. However, I think having to do an enormous overhaul of Sim personalities would probably involve extensive reworking of some of the game's core mechanics and I feel like that would also have to involve a period of downtime for the game. This is just from a layman's perspective, as I know absolutely nothing about game development.