Wait shouldn't bad luck and/or very poor decisions be what causes PC death?
Most of the time, certainly. But when you have the game rigged in the favor the PCs, those ARE the only things that will result in a PC death.
In AD&D, with save or die effects for example, a
single bad roll could kill your PC in the first round of an encounter against a creature appropriate for your level. Now, when people say "bad luck" IME they mean a
series of bad rolls, over the course of multiple rounds--the whole "Man, nothing is going my way!" sort of day.
And "very poor decisions" in 5E are harder to make IME than in AD&D. The entire mind-set has changed as well. Before you had to be careful to keep your PC alive, now you have to be truly reckless to have your PC
die.
That at least seems to be the majority sentiment in this thread.
That is because that is this is a 5E forum. So even though you tagged this as D&D General, you didn't put it in the older / other editions forum. What sort of responses where you expecting to get?
You won't find many "this challenge is too much for you" encounters in 5E, where running or surrender were your best hope and only insane luck would see you through the battle alive if you chose to fight.
By its very definition, a deadly encounter
might result in a PC death, but that comes with the unwritten extras that include resource depletion over prior encounters. It would be an interesting experiment in 5E to do nothing but deadly encounters, but allow PCs FULL LONG RESTS between every encounter. I doubt, again without repeated bad rolls or poor / wasteful feature use you would see many PCs dying after the first couple levels... even if then.
I think playing newer editions of D&D vs older requires different types of skilled play.
Your perrogative, of course. I think older D&D required more cautious and thoughtful play to be successful. 5E (and probably as far back as 3E really) doesn't require those. At most, it requires you remember how to use the features you have well--if you remember you have them at all.
I once played a Battle Master Fighter from level 1 to level 6 and
never, not once used a single battle master maneuver. Frankly, I never needed to initially, and eventually never bothered thinking of them at all. Could my PC have been even more effective at defeating enemies if I had? Certainly! But I didn't need to--it was overkill and would have made the game too easy for my enjoyment.
Personally, I will get rid of or nerf 90% of the features in the game, and not likely see increased PC death unless I ramped up the encounter difficulty from 2014 design.
An older player can join a 5E game and play "older style" and have their PC likely be fine. A newer player joining an AD&D game and playing "newer style" will most likely quickly see their PC die.