Requiring Castrum Litore completion for the next Bozja area is a bad decision.

[Gemina]

You can beat CLL with less people than 24 (I've cleared in a premade group of 16 + 3-4 pugs that joined when it popped in the early days of Bozja release), if everyone's on point and don't die to every aoe, essence's up and uses lost actions to supercharge everyone's damage output. You may not be saving every prisoner, but that's not mandatory to clear. I wouldn't be surprised if you could do it with 16 people, 24 is hardly the minimum. You only need 8 for Lyon + some remainder to handle the mobs in the main battle arena.

Also, while the superboss of each zone wasn't mandatory to progress the story of the zones and reach the next one, they were mandatory for the relic progression, much akin to how CLL will technically be gate-keeping the next part of the relic. And just like how they made it so that people could still complete the relic due to the Eureka zone bosses being impossible/nigh impossible to solo or low man, I have no doubts they'll do something to CLL to make sure it can be low-manned to future proof it. Just off the top of my head, they have these insanely powerful NPCs running around called the Blades, they could fit them into CLL and give them absurd stats to mow down the bosses if the game detects the player count is underneath a certain threshold. Not to mention possible 100% echo. No doubt they have something in mind with the literal terabytes to petabytes worth of metrics data they likely have showing Castrum on a decline.

Finally, I doubt Bozja will fully die out anyway, since CLL is currently the only source for the deep essence & bloodsucker/beast/whatever the healer one is essences, along with dropping Forgotten fragments that are extremely annoying to farm from star or rank V mobs. Which if the 48 man savage version is hard enough that it mandates those essences & actions for everyone, means CLL farming will become mandatory for anyone not willing to sink hundreds of thousands to millions of gil into forgotten fragments on the mb (seriously, I saw Forgotten fragments of Transcendence up to 10k/pop on some servers which gives you a 1/3rd chance of getting the essence you need) to get a good stock of the essences they need, especially if the savage version has as much replayability as BA did.

I'd wager that a good portion of players inside Bozja are not aware of even half of what you wrote here. Especially those who need a Castrum clear. The coordinated effort required for clearing with fewer players is a tall order, and cannot be expected from those who need Castrum for the clear. If players who have cleared it are running it, why would they be willing to forego even one prisoner? It is also risky as the loss to mettle should the instance fail is massive. Anyone less than rank 15 will feel this hit hard.

Just because it can be completed doesn't mean there are not fundamental problems with Castrum. I agree that I don't believe Bozja will die out. Bozja has incentive such as leveling and cluster farming. Castrum has little to none.