I don't agree with OP with gating expansions, but I seriously think they need to stop with the catch up mechanics and hand holding. Hand holding is fine, for the beginning while you're learning, but not through the whole game. Past level 40 tops, you should be able to figure things out without Blizzard holding your hand and guiding you. Ditch the catch up mechanics while we're at it. Make the content actually relevant again for the entire length of an expansion, not just 1 patch then, boom, useless as soon as the next patch comes out with it's tons of catch up mechanics to trivialize all new content. It's been that way since Cata and has only gotten worse each expansion.
Sure, TBC and WotLK had some gear catch up mechanics, but not many. You actually still have to go do previous tier content. That meant, there was always something to do. Back in those days, there were guilds in every instance. Some were hardcore and pushed forward to the cutting edge. Others took their time and worked gradually up, actually experiencing the content.
In the end, it's not the fault of the expansions, it's the fault of Blizzard for making all that content useless as soon as another patch drops. Hand everything to the players and what do the players really have left to strive for? Nothing.