Mobs can already demolish my mechanically solid high level bard in a few lucky hits, if I'm not prepared with stoneskin, and empowered endurance. Lowering the hp, we will definitely need to nerf spawns. Though, the best thing would be to -not to it at all-. Hp is already such a limiting factor, for all classes, except hp builds, which, generally are lacking in other areas. They have a lot of hp to absorb hits, but they take a lot of hits, whereas low hp characters have to make up other ways.
Curst Assassins. Those can instantly down a 140hp PC, assuming they land all their hits, and perhaps have a second assassin nearby. I've seen it happen before, even with high AC. We don't need less hp.