Having played a campaign up through level 20, my observation is that combat can be dangerous at any level. At higher levels it can slow down and get grindy, but it's still dangerous. I've got 3 clean kills among five characters playing from 1st through 20th level, evenly spread out across low, mid, and high levels. For me, that's about the right level of lethality. That said, if I wanted to make combat more swingy, I'd have PCs take average hit points and make monster hits do fixed but max damage. That would dial up the intensity.