In the Russian case, they're out of money. In China's case, the J-20 seems to be intended to utilize metamaterials so that the J-20 is not going to be Raptor-equivalent, especially when it uses EODAS like the F-35.And based on what?? They have barely designed and developed anything on their own, they have no engine nad if they continue with this political game I see no Chance of getting any help from anyone in the West, ... and meta-material is neither some sort of magic which solves everything nor is it easily available.
If it would be so much easy "to build a Raptor-equivalent considering that it's essentially 1995 technology" just tell me why Russia and China, both with vast experience and a well established aviation industry so far either failed or needed so Long to succeed and Turky with barely ZERO experience should manage this until 2028?? Forget it, this is nothing but a wet-dream and it will in the end even more dramatically fail than the Indian Tejas.