Heheh, I'll contribute to the minor rebellion since I like to see collectors in touch with what they're collecting, so it's not just a commodities market

I think AD&D in particular has an inherent power curve, namely Starting Ruin (level 1) -> Small Dungeons (1-3) -> Wilderness Adventures (3 to 8 ) -> Large Dungeons (4 to 8 ) -> Netherworld (8-12) -> Dimensional and Planar (9-25?).

You just need to be creative with high level characters.  They keep slaughtering dragons?  Have them be ambushed at night by two ancient magic-using red dragons flying invisibly over them while they sleep and using simultaneous breath weapons.  (That's a TPK for a lazy group but a huge challenge for a group that has put up sentries and magical wards.  Give them a chance but scare them good.)

Barbarian's unstoppable?  Have a riddle trap where a magic mouth compels the character with the lowest wisdom to answer, and if the player fails the character is possessed for 12 rounds and attacks his own party (favoring magic-users as targets, of course).

Party has too much magic?  Remember that items forged on the Prime Material Plane lose their plusses and powers, or become chaotic, the farther they go.  Have the party shift into another dimension, Castle Greyhawk style, and the only clue they get before battle that something transpired is "Your magic swords flicker strangely as you open the door."

Group has too many spells?  Give them an adventure in Limbo where spells that are cast are lost for 7 days, and can't be re-memorized.  (If you cast Fireball, no more Fireballs after the first one but you can memorize Lightning Bolt instead.)

Too much armor, weapons too deadly?  Pull an update of Dungeon of the Slave Lords and have them wake in a drowic prison with no gear and their clothing missing.

Think they can make every saving throw?  How about they wake after sleeping in a cave to a sound like the ocean, light their torches and see 800 large spiders descending from the ceiling?

Evil bad guy's minions:  Invisible level-draining undead with amulets of protection from turning.  Need I say more?

Too confident in general?  Recall Gygax talking about an encounter consisting of multiple liches armed with rods of cancellation.  Combine that with a time stop and the liches wanting to capture PCs alive for interrogation and see what happens.

The real question is just how much your players can deal with a devious DM.  Keep the rewards commensurate to the challenges, give skilled players a chance to survive and let the dice roll.  Spells like Wish and Resurrection are in the game for a reason after all.

If they're bored, give them Hell!  That's where Tiamat and her hundreds of children live by the way …

Disclaimer:  My friends have yet to forgive me for some encounters but they always smile when they berate me