This has always been a problem with FFXIV, but with every new expansion it gets worse. As per title, the amount of dead content is alarming, there are lots of activities that are impossibile to tackle for the average player (meaning anyone who doesn't have friends/a FC/is in a Discord channel). Variant and Criterion Dungeons, Eureka, Bozja, Castrum Lacus Litore, Delubrum Reginae, The Baldesion Arsenal, and of course older raid tiers and especially Savage.
Personally I don't have a solution, but a thing that I think everyone noticed is that every activity that grants a lot of EXP, no matter how long, bad or tedious it is, always manages to attract people. Maybe I'm wrong, but I don't think that the playerbase prefer grinding Bozja Southern Front FATEs than tackling on Variant Dungeons, it's just a matter of how efficient it is to level up alt Jobs.
What do you think? I discussed this with some friends and they say that these activities help keeping the social apparatus of the game alive. While I understand their reasoning I feel like sooner or later it's important to solve that problem.