The amount of dead content in the game is alarming

[Galvuu]

The solution is to make the content long-standing, and not just something that is isolated to that singular expansion.

Add regular updates to content, even if it is just additional loot, lore entries, achievements, EXP, or gear.

There is truth to what you're saying, but it's only half of the problem.
Some of the mentioned content isn't at cap, and, which each expansion, playing old content feels progressively worse. I ran a lot of Delubrum Reginae Savage in EW, and the latest BLM changes made what I used to play (Lost Flare Star Black Mage) horrible. Come 8.0, it wouldn't surprise me if, continuing current trends, Cirterion feels as awful as doing UCoB feels these days.
Until this game stops both disregarding low level gameplay, and accidentally gutting it with its ill-thought "reworks", a lot of this content will never see life even if they build on it beyond a small burst of activity as people trudge their way towards endgame content.

[KageTokage]

It doesn't help that moogle tomestone events actively discourage people from engaging with said content by providing easier access to their respective rewards.

[IvoryHawk]

I feel like The Baldesion Arsenal should now be taken out of the 2 fate system and put in like Delubrum Reginae savage, there's no need to go in and do the portal thing nowadays, especially when you hae FC's that want to participate but don't want to join large scale discords, just for a chance at a spot of getting in.