Let's look at a game world with a wide variety of levels realistically (well... we can't really look at it "realistically"... but at least "internally consistently"). Sure... there aren't many LG level 20 wizards out there, but when something big happens happens, say, level 12, what keeps the level 20 wizard from taking five minutes out of their day to 'port in and take care of it for the PC's? You have two ways about it: come up with a reason the level 20 wizard can't take care of it for the players (which means contriving events to fit the narrative [which I approve of as a DM tool]), or simply do what I do in my campaigns now: there are no allies above the party's level. There is no one to bail them out. Once they hit level 2, they became the go-to guys. It's the party saving the day, or it's no one. I have no problem with this (pretty blatant) tip of the hat to pure gamism and narrativism, since the very idea of "levelling" is already chucking verisimilitude out the window in favor of cinema.
Well, I'll take option 1.5: Remove teleport.
Much of the problems many have with high-level NPCs disappear once you remove teleport (and other means of near-instant travel) from the game. "What does Elminster do about this? He must be aware of this threat!" "I guess he is currently scrying us, and praying for our success - there's not much else he can do, being hundreds of miles away..."
Stationary portals can serve to make travelling the world faster without allowing people to pop in to any battle or dangerous situation.