Your suspension of disbelief has to be godly if you are alright with there being Super Mutants completely distinct from the Master. Vault 87 mutants, the institutes mutants, etc. If Bethesda wanted to bring Super Mutants back they should have had surviving members of the unity continue the Masters work. EDIT: Wow, I guess I was tired when I wrote this, I forgot to give a good reason. My reasoning is that it just seems highly unlikely that people would get nearly the exact same result of creating Super Mutants like the Master did, I can believe others being able to create something like the charger from Left 4 Dead 2, but not something that has the intelligence to pick up a gun and shoot it. It also makes the Master look cheap and unoriginal seeing as there are other people entirely distinct from him making them.