That's the thing.
Take when Northstar & Aurora became elves rather than mutants.
How stupid was that?
How annoyed were we all?
Imagine that times hundreds of thousands.
That's what would happen if Marvel made every mutant an Inhuman.
Sales would drop.
Let's flip it:-
Sasquatch, Shaman, Snowbird are suddenly Inhumans or Mutants.
How would you all feel about that?