The almighty grindmill and its junky slaves is the whole problem here. If they don't add T4+ places with appropriate loot, those T4 hamsters (whose only motivation to play any game seems to be "get moar l00tz")* will complain about lack of incentive to play. If they do add T4+ places with appropriate loot, they'll be 1) trivializing even more of their old content (thus actually decreasing total "usable" content despite having added a brand new raid) 2) increasing the already massive difference between new and old players further.
A T4 clad, AAed out hamster is already virtually lv86, for whom almost all content is trivial. If there's a new T5 this hidden level cap will go even higher. How exciting and fun is it to do Halls of Frost (L72) with a full group of 80s? As exciting as doing Atzel's Fortress (L80) with a full group of T4 hamsters. T5 will surely atzelize Khitai and T3 (if they aren't already).
FC is damned if they do and damned if they don't. Unfortunately, their best bet is damnation by doing and adding T5 and further to keep those hamsters running. It's those hamsters who bring in the subs after all.
*There are some raiders who understand that the one and only point of having better gear is to be able to fight against bigger foes, but they're a depressingly small minority. Yes, I'm aware of the irony of this statement.