WotLK was the golden age.
The last two expansions (Cataclysm and MoP) included several attempt to go back to TBC model and failed miserably.
Actually I felt Wrath was more like an extension of BC, from 5-mans to 10-mans to tokens for gear.
Cata was when it was starting to feel like same old, same old with the mob health and damage scaled up. I think people tried to find different outlets like in alts but it didn't last for long and they unsubbed.
MoP is a decent xpac but it does feel like Cata in the sense that we've seen it all before. Cata left its mark and changed how many people see the game. If MoP or something like it had been released instead of Cata, the sub numbers might have continued to go much higher before gradually (and inevitably) falling.
I think the big problem in Cata and MoP was and is catering to those asking for harder content.
Cataclysm dungeons drove away a lot of people, and MoP grinds and hard raids made around half the raiders to stop raiding.
The fact that MoP doesnt have catch up mechanism for those that couldnt complete the overtuned tier 14 only made it worse.
Blizzard needs to go back to the golden age of WotLK, make normal raiding accesible for raiders, provide catch up mechanism and develop new dungeons and raids.
The problem with Cata wasnt catch up dungeons, nor tabards, nor anything like that.
The problem with Cata was that IT HAD LITTLE AMOUNT OF CONTENT (probably due to the huge amount of resources used to revamp old world) and the content it had was badly tuned or was just low quality.
Tabards and catch up dungeons worked great in the best expansion WoW had, so they were clearly not the problem.