It's not just firing them or taking back their options. The incentives change such that the company doesn't really have to put any effort in retaining them anymore. Bonuses/raises/soft perks/etc that the company might normally provide would be less likely. They don't necessarily have to worry about keeping you happy with what you're working on which is not something you normally think about when considering the value of equity.

It's hard to predict exactly what will happen, but the broken incentives are clear. A better model would have them aligned much better.