I'm surprised no one here is talking about why Paradox still hard locks DOTF to only Christian and Islamic faiths. Sure, some of the eastern faiths, like Confucianism/Chinese folk-religion didn't historically practice heavy proselytization or militarily defending their faith, but it would be nice to at least have the option to use the mechanic even just for balance reasons. The extra 5% morale provided by DOTF can turn around decisive battles in grueling wars, and the inadvertent nerfing of ROTW faiths is really unnecessary.
For fantasy/alt-history situations, why shouldn't a resurgent Zoroastrian Persia, a Jewish Ethiopia, a Totemist Pan-North American Native Empire, or a heavily militarized Buddhist Khmer invading the Middle East/Europe be unable to declare itself the defender of its state-sponsored faith, the way Christian and Islamic faiths can?
If the issue is AI abuse, just make it a player only decision (AI = no). Perhaps certain conditions could be added to require taking decisions with diplomatic consequences, or holding certain provinces deemed holy for your faith before a ROTW faith could claim DOTF, but come on man, give us the option to do this.
Hasn't Paradox learned from CK2's infamous "Will not marry an infidel" malus, that no one likes hard locks/restrictions? Soft locks with a challenge are almost always preferred to allow more options/playstyles.