I'm sick of people stuck in AD&D mode freaking out at versatility. Essentially you are saying that any character that can surprise you with a single spell a day, will throw off your DM-FU,
leading to cats and dogs, living together ....
total anarchy!

Not what I am saying
at all but I applaud your impersonation.
Nope. Once you get to 5th level, it really isn't.
My then 6th level Fighter failed a DC 13 Con save and started dying. Twice.
The first time on round One.
So, was this a surprise combat? You had no idea you were heading against the creature? Oh well, tough.
Hmm... CON 14 or so (typical of level 6 fighter, but you might have even has a better CON) and proficiency bonus +3 means you had a 35% chance of failing this save. For that to happen
twice is about a 1 in 8 chance. But hey, let's not worry about the randomness of the game.
You were
dying. Big whoop... Even without help you have a 60% of making all your death saves and stabilizing on your own. Dying =/= Dead.
A 10th level Halfling Bard with a +4 and Advantage to a DC 12 Suggestion spell, rolled the number two twice, and failed. The Suggestion, essentially took her out for the day, due to the spell duration.
In the same session, the remaking three party members fight 4 trolls.
Oh, come on! You're a bard! What was the suggestion-- "Don't dispel this magic I'm casting on you, and don't let anyone else do it?" I mean, you're talking a 2nd level spell so Dispel Magic automatically cancels it.
And 4 trolls against a party of 3 10th level PCs is hardly a difficult fight IME... but without knowing the particulars... who can say?
The monk is knocked unconscious by Troll 1 of 2 trolls surrounding her.
Troll 2 moves into feast. Each attack has Advantage, each hit is an Automatic critical hit, and more importantly, each hit counts as TWO Death Save Failures.
(Two hits and the character is Dead.)
I rolled two sets of the number 1, twice.
AWESOME! I am glad your PC died. I never said it doesn't happen, but the game is geared for the PCs to win.
Oh, and was the monk using Patient Defense? Did she realize perhaps Disengaging would be better?
Again, you can give all the bad roll situations you want, but I've seen that often when PCs do die, it was because the players acted foolishly (just to be clear, I am not assuming you acted foolishly because I don't know the whole scenario, etc.). Oh, and even then, a simple
Revivify and you are alive again--let's not forget that.

Or, at this level you probably have access (directly or not) to a raise dead and/or reincarnate.
I had the troll pick her up and take her deeper into the caves, (with more trolls), with the party and other trolls chasing behind. We were playing on 9/11, and as a group we have been referring to it as the 9/11/2020 Miracle.
YEAH, the PCs win-- again. Big surprise, huh?
Was this the
same 4 trolls from the earlier battle? Again, so much depends on how you play and how prepared your group is, etc. The battles don't sound that difficult to me, more like improper preparation and tactics (and sure, some bad rolls, apparently all in one battle).
In our last session the party almost faced two TPKs, but it was because the players were
NOT making good choices. I didn't want to kill them, but like you said, the dice fall as they may.

In the first, the Paladin died and was turned into a Shadow (so nothing short of True Resurrection or Wish for him) and in the second the party only survived after the Barbarian/Fighter used inspiration and indomitable twice to keep raging.
The sad thing is: neither battle should have gotten to the point it did. It was flat-out bad choices by the players. One player looked
aghast when it seemed like it was over in the second fight. I wasn't going to remind him of indomitable--after all that is his responsibility as the player-- but after a moment of frantic searching he did.
Anyway... DM fiat can make the game tough or deadly, but again --by design-- it isn't. FWIW, 4 10th level PCs against 4 trolls
IS a deadly encounter according to the design of the game... guess in your case (due to some bad luck) it finally worked out as designed, huh?
