They don’t need to do “all the content” or get “all the gear,” and that’s not even realistic to how the game works currently. TF, much as I hate it, is still very infrequent (especially compared to the amount of hate it gets).
But raids make up the primary story. They end the storylines of zones. They are the conclusion.
If they want to gate that behind difficulty that’s a very silly thing to do. Gear/prestige? Fine. Story? Not okay. And going to third party sites to “watch” the ending is an absurd suggestion. People play the game to play the game, not tab out and watch something so they get some clsure.