"If this problem is so bad, why aren't YOU doing it, NPC?"
My stock answers:
NPC #1: "Because there is an even worse problem, and I don't have time to do both."
NPC #1: "Things have gotten worse, I need you to handle this other small problem for me."
NPC #2: "NPC #1 died trying to handle the problem. Oh where are we going to find a hero the like of NPC #1?"
NPC #2: "Help us PC's, you are our only hope!"
Hopefully this eventually leads to:
PC #1: "I can't do two things at once. NPC #3, you are going to have to take care of this small problem by yourself."
I want to note that this does not 'deprotagonize' the PCs IMO. At each stage, they are percieved as 'the hero' by those below them that they help and eventually they graduate to being 'the hero' for everyone inheriting and eventually transcending those that from their initial perspective seemed so powerful and heroic.
It's like being told, "You are a greater hero than Batman ever was." You can't do that unless you have a 'Batman' to measure yourself by.
There are a few 'nuclear missiles' (as someone called them) in my campaign world who probably would help in event of end of the world threatening destruction: Jace Merlkin the Dragon Hunter, King Averthas, The Lord of Dee, the Provost Council, the Master of the Mystic Isle, etc. but I don't really know that 'world threatening destruction' is my style of play. The only real 'epic level' play I've done was as a PC. It takes years and years of play to get up to 'save the world level' and I've moved around alot IRL. I'm not even sure what the 'save the world' level threats would be. I know several ways you could literally destroy the world - cut the Orichalcum Thread, recover the 'Power Word: Apocalypse' spell, return the Art Mages from their imprisonment - but not really who the nemesis's would be who would set the 'end of the world' scenario in motion. I'd probably invent a nemesis to suit the players if it came to that, but at this point in my life I doubt I'll ever spend 2000 hours playing one campaign again. Heck, at this point in my life, I'd settle for 20.
In any event, if the real problem with a 'destroy the world' plot isn't so much why aren't the NPC's helping, but that the gods would almost certainly intervene directly before it reached a certain stage. Which transforms every practical 'destroy the world' plot into some variation of 'Trigger a breaking of The Compact' and thereby destroying the world, because any plot that didn't threaten The Compact would just get outright squashed (see the aforementioned Art Mages).
Actually, now that I think of it, this might be the reason I've always shied away from a 'end of the world' scenario. For any given scenario, the real problem is, "If this problem is so bad, why aren't YOU doing it, Divine NPC?" At least twice in the past the dieties have joined forces to prevent the destruction of the world, simply because even most of the evil deities want to rule the world not destroy it. I have a hard time envisioning how any destroy the world plot can get around that basic problem. However, it's then easy to explain why gods don't directly intervene alot in smaller problems - they don't want to escalate the problem to a 'destroy the world' crisis.