In the USA FDA would need to approve such a change for a device that could possibly kill the patient. And would be unlikely to say yes.

I once designed a hardware state machine out of TTL (no CPU, except a network interface that could only passively read and transmit the state) to ensure that a pressure vessel was safe (and this was ahead of the mechanical safety devices). Regularly someone would ask why we had this large device and didn't just replace it with a tiny MPU.