I think it's a good thing that 5E doesn't rely on save or die. On the other hand, PCs are no more or less likely to die (or come close to dying) in my games than in at least the last 3 editions.
At higher levels, I've found it difficult to challenge players in every edition. There's never been an edition that I haven't had encounters that I thought would be incredibly difficult turn out to be a cakewalk.
Want to put the fear of god into players? Throw a roomful of flameskulls floating around starting out at max fireball range. In the dark, when at least 1 member of the party needs light. They're only CR 4, right? Have a purple worm erupt out the ground, swallow a PC and then finish it's movement underground. Have any number of incorporeal creatures that life drain pop in and out of the room, slowly eating away at those max HP.
The number of ways to threaten a party is nearly limitless. Sometimes you want to throw Tucker's Kobolds at them, other times just throw a couple dozen hill giants that are actually using reasonable military tactics like having spotters and giants throwing boulders from behind total cover. Heck, maybe your world has become the multi-verse spanning spawning ground for Tarrasques.
Normally there's only one, but baby Tarrasques have to come from somewhere, right?
P.S. I've the things above at one time or other except the Tarrasque breeding grounds. I'll have to think about that one.
