I totally agree with you, but you need to see it from a developer's perspective. I put a lot of work into D1, D2 and D3 and hate the idea that once players reach a certain level, all the work we put into terraining, designing and stuff is totally pointless anymore, as nobody wants to go inside those dungeons a second time.
Lore-wise, level 50 bandits wouldn't make sense. But from a gameplay perspective, this is a step we have to consider in order to increase replay value. And for maps in warcraft III, replay value is the most important thing.
To make it true-to-lore, you could use the existing dungeons for high level quests.
An example could be something like this: The player speaks to a captain in Mytargas, he tells the player about a traitor who planned to overthrow the current "people in charge" before he was discovered. He has fled, apparently hiding somewhere close to Riversdale. The quest is to talk to riversdale townsfolk. The Player, talking to the townsfolk, then learns that the guy and his most trusted allies were seen heading for the bandit caves (Dungeon 1).
At the railroad part of D1, the place near where the final boss is currently at, there is a pretty large area to the left not used by any mobs or anything. In this area, a neutral bandit npc will be standing. If the player, being at least lvl 35, talks to the bandit, he gives a little speech about how stupid it was of the player to come - that sort of bad-guy thing. After the speech, two other bandits are summoned (they had been hiding), one a healer, one a damage dealer (the last one would then be a tank), and they are all bosses. A tough fight will start, and, after the bandits are dead, the player goes back to Mytargas to claim the reward.
The bandits should of course drop bossworthy stuff for high levels since they are pretty tough to kill. But since they are not part of a full high level dungeon (high leveled players can just rush through the dungeon to sergeant and then kill the remaining 3 mob groups before going for the 3 bandit bosses) the gear they drop shouldnt be the ultimatively best, only second best.
Finally, to make the quest fit into the gameplay of farming for items, people who have already completed the quest can still trigger the dialog and start the fight again. People who are lower than lvl 35, and who have not completed the quest first (unless they are at the stage where you are send to kill them), they cannot trigger the fight and the bandit will just tell the player to "piss off kid" while being invulnerable.
EDIT: I also have an idea for other quests like this, but i dont want to write a wall of text unless you ask me to