KahlessNestor
Adventurer
This is the same as mentioned above where Thor or the rest of the avengers are mysteriously absent from a world altering event with no explanation given. The just-shrug-and-ignore-it excuse is extremely lazy writing and bugs me in comics, movies and any other media.
One of the things about running an RPG is that you can have the characters react the way you want. The whole deal where you are yelling at the screen "No! Don't split up again are you stupid?! Someone dies every time you do that!" And then you get frustrated and angry and never watch that show again.
And yet I see people posting events in their game where they do exactly that because "Yes, I know it is a bad idea, but it is so classic to the genre I'm going to do it anyway." No! It is not classic! It is lazy writing! The author couldn't think of a good reason for the group to split, so he just had them randomly decide to do so for no reason, just to move the plot along. Arggh! It makes me crazy!
Sorry, I kinda got off track there... Back to the point...
Sure, you can ignore the obvious solutions to a problem "because they are too easy and don't tell a good story". And if your group is fine with that then of course there is no such thing as bad-wrong-fun. But for some of us if you want a good story, then make sure you have a good reason for us to not take the easy solution.
You have a have a bunch of Demon Lords invading the Underdark? Well we have a scroll of sending and my wizard met Khelben "Blackstaff" Arunsun as part of his background. Why wouldn't I just contact him and let him know? Why wouldn't he send a warning to Elminster? By the time of the final showdown I expect that all of the high-powered casters in the realms, along with their favorite meat shields, will be there to kick some serious ass.
TL;DR: I'm not going to do something stupid just to save your plot. But this gets to your next point.
Yep, you need a reason for why the heavy hitters are not going to bother saving the world. And the reason it is a valid criticism is that the published adventure paths don't seem to have any reason for them not to. The fact that Elminster is not even a consideration in the Tyranny of Dragons adventure line is the problem.
Honestly Elminster is always a problem. Now he has the same excuse as Superman or Thor, and that is that he can't be bothered by local events, and that makes sense. But for world shattering events there needs a reason. The usual cop out is "He is away on another plane", but that gets old after awhile.
Or maybe he was dealing with another Cult of the Dragon issue in the Dalelands and wasn't able to help?
Also, Khelban is dead by the time of the demon invasion of the Underdark.