I hate to interrupt the discussion of Gods and Powers, but I had a discussion with a friend (a huge FR buff) who suggested something interesting...
We were talking about Curse of Strahd and, how except for minor differences, Strahd is just a MM spellcaster-variant vampire (with a higher Int score a few different spells) and Mordenkainen was a MM archmage (rather than a unique NPC stat block). Accerack in ToH is a MM demi-lich as well (with his variant power in the MM). It appears that, with a few exceptions, WotC ok with not custom crafting stat blocks for NPCs and is willing to use MM stat blocks for famous heroes and monsters. This lead us to a thought about how other "NPCs" in the Realms might now be as powerful as they were made before.
What if, Elminster is an MM archmage rather than a 30+ level fighter/cleric/thief/wizard/archmage that 3e had him as? What is Artemis is a 16th level rogue (assassin), Drizzt a 16th level ranger (hunter)? The Seven Sisters weren't epic level but Challenge 12-14 NPCs?
What if the "epic demigods of FR" aren't so mechanically epic anymore? That a group of mid-levels PCs could take out Elminster or Drizzt? They are high levels for sure, and Elminster is still capable of 9th level magic, but They aren't the Epic Level Gods of 2e or 3e anymore? (Most of their story exploits being owed to plot-armor and convenience) Thus, we can answer why can't Elminster go and solo Tiamat in Rise of Tiamat? Well, can a challenge 12 take out a challenge 30 solo by himself? Grab the stat blocks and try it. Hell, give Elminster help from Kelban and Allustrial (MM archmages) as allies and have them take on Demogorgon (Challenge 25, OOtA) and see how that combat plays.
Once you frame the notion like that, I wonder if it changes the notion of "why isn't Elminster stopping the Underdark invasion of demons" when he's more limited to what a 20th level wizard can do rather than a 30th level rules monstrosity...