I agree but see that's where the module descriptions are being over dramatic. I very much doubt the world is remotely facing destruction. There have been many demon and devil incursions into the Realms and most of them count as local issues. Now not knowing the details of this adventure I'm not sure how bad it could really get but I assume that it will all be dealt with as it has been countless times before - a bunch of local heroes will go and destroy the bad guys and seal the gate. The only reason Drizzt is even part of this one is because of the novel tie in, but he tends to busy himself fighting orcs, giants and drow. Hardly the world saving type really.
In the Dragon Queen story line the outcome could have been a problem for the Realms considering she is a Goddess but the party and their army became the best prepared to deal with it by the time it became an issue. The princes of elemental evil are relatively weak and pose no more than a local threat, even with the cults
The high level Realms characters aren't all powerful and do not adventure as such. A bunch of mid to high level player characters are just as capable of dealing with some demons as Elminster is - probably more so. Elminster just knows lots of things and many secrets. He isn't anything particularly anything special in combat. At leas that's how the novels tend to portray those characters. He's much busier trying to influence nations and repair the weave.
The DMG tells us that at 11th level characters are masters of the realm and at 17th masters of the world. This isn't true for the Forgotten Realms - its too big for that. I would say you would be known in your region but not much beyond that. So there are lots of high level characters in the Realms - both PC''s and NPC's. There is still plenty of space for the lower level heroes to do their bit