Yeah, I really DON'T get why they haven't done this. They use stuff like Relics or Moogle Tomes to get people to do old content when they could just...make it worth doing. Why don't ARR FATEs give Bicolor gemstones at this point? All FATE content is timed and synced, so none are "better" for this than others. And the fact ShB ones still give them means it isn't "to get people out into the new zones". They could add old zone stuff like riding maps and orchestrion rolls to them easy enough with almost no effort. They could add almost nothing at all (they'd probably want at least a City-State vendor in ARR for players that aren't to ShB zones yet to be able to spend them).
Roulettes are also untapped. Let people roulette higher floors of Deep Dungeons, and hell, que for them without having to go to the location. Part of my old school MMO brain hates that, but it's not like they aren't ques as-is, you aren't physically walking from Quarrymill to Palace of the Dead's overworld entrance anyway. Likewise, why isn't there a Criterion que? Why isn't there a Rival Wings Roulette? Throw Delubrium into...something.
These systems already exist. There's little to no "new" programming or content that needs to be added to them. Even just the bicolor gems would be a useful change and there's straight up no reason not to. There are so many "dead" systems in the game that they've had that were good or could have used some tweaks, but they're just extinct content at this point when the most minor of changes would send engagement up.
EDIT:
Sorry, post limit:
Yeah, but this is already true.
Moreover, we're talking FATEs. They spawn on a timer, so you can't farm "only one" efficiently. The most efficient method of farming is just to move from FATE to FATE as they spawn. There are the big event ones (the 30 min duration ones with the rare drops/rewards), but outside of those, there's no real harm to the system.
For bicolor gems, anyway.
Roulette rewards are also only once daily, so again, there's no chain spamming for efficiency.
Heck, even Moogle Tomes. I've seen more people doing Rival Wings - RIVAL WINGS - than P1N in Savage BiS. A lot of players don't HAVE Savage BiS.
I'm not saying no one is, but people seem to be finding other content, including stuff rarely done, to get Moogle Tomes with. I don't think that example is helping you. I think maybe tryhard/hardcore/elitists do what you say, but everyone else - which is a lot more of the playerbase - isn't doing that. They're doing what they find the most fun/enjoyable of the things to do.
Besides: Those people doing that BiS spam stuff are already doing that anyway. There's literally no stopping them and there's no system you could design that doesn't have one way which is more efficient than others. That doesn't strike me as a good argument for not having options. Even if fewer people are doing the other options, if it takes literally nothing from the Dev team to enable them other than a few flags on content, it's totally worth it.