Cores can be an abstraction based on historical claim to and ownership of the province without being something historic themselves. CBs are justification in the eyes of the world. Surely you don't think you should get the advantages of a reconquest war with a fake Catholic Persia?
but what makes a core from a fake catholic persia, any less of a legitimate cause for war, than a fake fabricated claim on a piece of land?
We're likely changing it to be based on majority religion of cores. Will count non owned cores too, meaning no releasing 3 development Catholic Persia and feeding it all its cores.
the problem with this, is that it requires you to take and convert at least half the land (and probably core it too, before you can release it) which is pointless, after you have converted and cored it, why bother releasing it.
Eventually, states acquire the ability to set up legitimate states that have no basis in tradition - it's called Client States at Diplomacy 22.
this argument is about as good as the "coring cost too high? wait until Admin Eff kicks in to expand" argument. I should not have to wait until tech 22 to be able to create useful vassals.
Sidestepping the choice by allowing you to farm out your conversion tasks to vassals undercuts several of the options and generally removes an interesting choice and an intended tradeoff.
its sharing the load, and it doesnt come without issues, you still have to babysit your vassal and deal with their rebels. its not like you could not convert the land yourself but when you are taking large swathes of land, it makes it more difficult. (incoming argument "well dont take large amounts of land")
At the end of the day, a vassal is a subject of the overlord, and the fact that you made it possible to enforce religion on same religion group suggest you believe that the overlord should have control over the vassals religion. so why stop it? most people like more ways to play the game not less, if you want to make it more difficult for us to make same religion vassals do it, if you think we shouldnt be able to do reconquest on catholic persia's shiite cores, then perhaps make it increased AE or something, , but dont remove the option entirely.