Should a GM let high level characters die?
I certainly would let them die, if that's the natural outcome of play. But old school D&D rules tend to make high level PCs robust, and also tend to make the players of high level PCs highly skilled! So it's not a common event. And it tends to be a dramatic, epochal event when it happens.
I think it was in 2016 I last saw a really high level PC perma-death: At the climax of my BECMI campaign, my son's PC the Duke-Sultan William of Castellan/Karameikos/Alasiya, an MU 18 (boosted to 19th caster via Ioun stone) was pressing home the attack against the Heldannic Knights during the Soderfjord invasion of Ostland. He had just nailed the Heldannic Knights-Marine with a well-placed fireball as their Warbird's bay doors opened, saving his party from being overrun by a couple dozen high level Fighters, when he was Disintegrated by the Heldannic Warbird's Bile Belcher cannon. AIR he was targetted 3 in 6 and would have survived on a roll of 4+ on d20, but made me roll and I got a 2. William was gone, after years of play (he had started at 4th around 4 years previously, when my son was only 5 years old!) but his side was victorious, King Hord Darkeye of Ostland was defeated, his kingdom conquered, and William was mourned by an assembly of high level heroes, including his son Bravery the new Duke-Sultan, the High Priestess Lady Roseanna and Baroness Alexandra Vorloi, all great champions in their own right.
In that case I was already planning to end the campaign with that session, although I did end up doing some epilogue stuff.
No very high level (15+) PCs have perma-died in my 5e Wilderlands sandbox game yet; High Priest Thuruar (Clr-15) did perish to the hellfire ball of the Death Knight Varek Tigerclaw*, but Hakeem the Barbarian defeated Varek and was able to get Thuruar raised by Lady Meda of Thusia. If a really major PC like Hakeem (now Barbarian-20) did die it would certainly alter the campaign (Hakeem's newly founded empire would almost certainly collapse, for a start) - but it certainly wouldn't end the campaign.
*A former PC reanimated by the Necromancer Borritt Crowfinger, BBEG of the campaign for many years until his final demise a few months ago.
Normally players & PCs are happy to team up for adventure, whoever's idea it is, as there are rewards in fun, gold, XP etc. But I do see stuff vetoed, eg the PC Hakeem refused Shieldbiter's request to help him go up against the ancient red dragons beneath Fortress Badabaskor, rumoured to possess the sacred Dragon Armour and Dragon Shield of ancient Arkhosia.
Edit: Rely on coincidence - yes just like in the comics (Savage Sword of Conan, notably) and pulps, heroes are always running into each other in unlikely circumstances. But I also have a home base, Selatine port village, that serves as the heart of the campaign, the starting point for (most) new PCs & PC groups, etc. With lots of PCs based there they naturally interact and form adventurer parties.
Oh, also re the tabletop element, I would be willing to run up to two separate groups on alternate weeks if necessary, if the PCs split up. And I have a co-GM who helps out, running the Bratanis region, a low level campaign area west of Selatine. PCs can go back and forth from there, being GM'd by him one week then me the next.
Bill also lost his level 8 Dragonborn Fighter Drakonok to a horde of mook orcs a few weeks ago - turns out that in 5e challenging an entire orc tribe to battle is not such a great idea. Mind you when Drakonok's companions (who had fled the orc horde summoned by Drakonok's roared challenge) came back across the scene an hour or so later, the breath-frozen, stabbed, and arrow-pierced orc bodies were piled high in the corridors of Stonehell.
In a single-party campaign, the group wipes, I may use that setting again later. Again BBEG victory will change the campaign world but it likely is still useable. I certainly won't fudge to keep them alive. It makes the game meaningless if the PCs can't lose.
In an Adventure Path type game everyone plays the same one PC to the end of the campaign.
The two cases I recall:
1. Rise of the Runelords AP - PCs were wiped out by Nualia at the end of Book 1 ca level 4, resulting in the destruction of Sandpoint town, the intended home base for the campaign. Years later Sandpoint was a goblin-infested charred ruin. Eventually the Black Dogs mercenary company was hired by Titus Scarnetti, one of the few survivors of Sandpoint, to clear the ruins - this went on mostly in the background of the new campaign though the new PCs did interact with Titus; Quillax the Druidess even delivered his wife's baby and ensured the child was born without the taint of Lamashtu (his wife & daughter had been captured by Nualia's Lamashtu ("mother of monsters") cult but ransomed back, so there was a fear they could have been infected).
2. My Wilderlands - catastrophic PC defeat at 10th level, in the final battle the lead PC Varek Tigerclaw fell in the doomed defence of Bisgen town against the Necromancers of the Black Sun; leading to the rise of the evil Empire of Neo-Nerath. Subsequent PCs spent years battling the Black Sun as it embarked on a genocidal campaign of conquest; at one point the new hero Hakeem Greywolf battled Varek Tigerclaw, now a Death Knight of the Black Sun - and ripped out his black diamond heart.
*An unfortunate Paladin.
It was definitely bathetic - and AIR I did have the PC resurrected some years later to continue service to his dark master. But the old stories have loads of bathetic ends. Jack Vance is inspirational. The PC Mortis was ultimately executed by Graz'zt when he (a) started to get cocky, beginning to see himself as Graz'zt's equal, and (b) accidentally violated a non-aggression pact Graz'zt had with Thrin, a lesser god lawful good PC.
I don't make assassins omniscient. I recall one time a Neo-Nerath assassin struck as the ca 10th level PCs were resting at a mage conclave on their way to close the Black Sun gate. With the PC group was an NPC (from The Licheway, I recall) who claimed to be an Archmage, he dressed the part but was really a phony, a 2 hd non-caster. Anyway, when the Black Sun assassin was choosing his target, naturally his top priority was to take out the enemy Archmage... ...And the PCs slept peacefully all night.
I generally find player characters with powerful enemies do take precautions, and aside from 3e/Pathfinder I can usually have those precautions be effective, or dice for it (3e/PF is broken IME and often requires handwavium why high level caster NPCs don't use scry-and-die on the PCs). I recall one serious assassination attempt - the PCs were in Ahyf, local hive of scum and villainy, and got into a big fight at the fighting pits liberating some pit fighters. Hakeem was badly wounded killing Gorok Halfogre the pitmaster, giving three Black Sun assassins who had infiltrated the town a chance to strike when he & his friends fled town. It resulted in a brutal night battle on the road but the PCs managed to prevail and took shelter in a farmhouse, fearing more assassins.
If Hakeem or similar high-value PC was foolish enough to put themselves in a position where an assassin could strike as they slept, I would roll with it, using the 5e rules they would take a critical hit in damage, plus poison - likely would kill a Wizard PC or similar squishy, but a high level Barbarian or similar might well survive the wound, wake up and kill their attacker. It would take a lot of CR 8 Assassins to take out Hakeem, 20th level Barbarian with 2 Epic Boons inc Epic Fortitude, CON 24 & 325 hp (takes half damage from weapons while Raging, so effective 650!). My son Bill's Dragonborn Barb-17 Shieldbiter with 209 hp might be more vulnerable, but generally it's the high level casters who really need to worry.