Yet they managed to do it in vanilla and TBC. The answer is a) to make sure there is a decent amount of content, b) that the players aren't able to burn through the content too fast by providing actual and adequate challenge on it and c) to provide an actual fun, deep and interesting battle system, suited for group play.
In legion there is a decent amount of content, but you typically run over it way too fast and the battle system isn't at all suited for PVE group play. Blizzards tries to offset this with a so called gear progression (eg making you grind for something you had baseline/way faster during the older expansions), but only the less exigents players will be satisfied with that.
Every dungeon raid and dungeon of legion should have been like karazhan to make it interesting.
The issue with the replayability of the dungeons has more to do with how the battle system has been tuned since WOTLK (very little emphasis on player coordination and cooperation in the use of their abilities) than the "difficulty". Back in vanilla and TBC you had to adapt your gameplay to the players of your group, as you do in a game such as league of legends. You also had to strategize before most bosses. Since WOTLK dungeons are a matter of AOEing mobs to death while having a relevant ilvl, no mater what your group composition is, as such the replayability is very low (as you re doing the same thing no matter who you re playing with, which is a huge design flaw for a portion of the game based on group play).It drives me crazy to see people cry about replayability of dungeons when one of the biggest concerns was how easily outdated they got?
The only way to satisfy you seems to release 10 dungeons a month.
Redoing content has been a part of MMO's since the first one was released. If you don't like this, then maybe you should stop playing MMO.