"Because they are low stress and low heat due to oil cooling...

"Because they are low stress and low heat due to oil cooling there will b a suitable low cost option that will perform suitably."
Guessing you are saying low stress due to the braking force being applied over a number of disks which therefore reduces the stress compared to just being applied over one disk.
The oil gets hot , above a certain limit in temperature performance and or excessive wear/breakage will start to occur for brake components , they will have to run tests to find that limit and if lower than metal rotors will need to find a way to increase cooling , ie a heat sink or radiator or fins on castings I guess.