Since it is the main mirror that is flawed, then the light collecting ability of HST is flawed. Any and all light that reaches the instruments would be contaminated so probably everything needs the Corrective Optics (COSTAR) in place. Removing COSTAR because new instruments don't need it might complicate a servicing mission too. And there would be the risk of confusing the instrument designers into building an instrument that is supposed to work *with* the corrective optics in place. You might get a situation where an experiment fails because one instrument uses metric, and another uses british units.