I think the difference here is that doing so would be a hardfork and would require everyone to opt into the change. Contrast this with PoS where if 51% of the stakers woke up one morning and decided to rewrite the last 2 weeks of the chain they can immediately do so, and anyone who disagrees must opt out.
With slashing PoS, if stakers tried to rewrite the left 2 weeks of the chain they could have their stake taken away as a penalty for signing votes on multiple forks.
