But making content that would only appeal to large FC's WOULD be making content that is limited to a certain group of players. You would also need to think about the gear that a 24 person raid would reward. If you make it the same as 8 person content, then many people and FC's would not find it worthy of doing, and the dungeon/fight loses its relevance. If you make the rewards better than its 8 person equivalent, then it would be a roadblock for people in order to acquire better gear because not everyone is part of and doesn't want to be a member of a large FC. Yes, there are fights like T5 that don't appeal to every player, but you do not need to be a member of a large FC to do them.
When you make content that can be scaled to 8 and 24 player versions, you also lose creative room because you start fitting mechanics that would be able to be used in both versions, therefore constricting the potential of content.
There are games that FFXIV could rip some pages from, but please don't use WoW's raiding scene as an example. Every MMO that tries to copy WoW fails because it loses its identity and becomes a mimic that doesn't stand up to the original. There are also a lot of different gear variants available in WoW per patch, and along with 4 or 5 variations of the same content/fight, it just makes things annoying. Right now, if you want i80 gear, you can do Ultima or CT. If you want i90, you can do EX Primals, get Myth, or BC. If you want i95 weapons, you can do T5. Now imagine making 4 different variations of Allagan weapons, with 8 person, 24 person, 8 person hard, 24 person hard (that's what WoW is), and it just becomes redundant.
Organizing serious 24 player content can also take some time and would only add more waiting to the already-present waiting room that hitting level 50 becomes. I am not against this type of content, but I'm saying that SE should be really careful with this in order to not have the oversaturation of gear and content redundancy that WoW has brought upon itself.