Hmm. This is all speculation, but here's what I think: while traits might influence a townie's life, they're
not the biggest deciding factor in what happens to them. The randomness of Story Progression is. I've had a Sim with the Commitment Issues and Dislikes Children traits get married and have twin girls under Story Progression in the past. Furthermore, I don't think townies will take initiative to switch their traits around. A while ago LlamaMama came up with a great
Schrödinger's Cat analogy: Townie Sims are pretty much sealed into a box when you're not playing them, and don't actually act like "normal" Sims until you drop by the visit them. When you're not directly observing them (ie. in their house), townie Sims are stuck in Limbo with the occasional birth, death, skill, job, move or relationship. The game just uses as little data as possible for the townie Sims, so they're very unlikely to start building Time Machines behind your back.