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Changing the sensors would make it very hard to evaluate under the very strict condition they were tested. The system integration would be very hard.

The sensors on JWST have are outdated and do have some issues. For instance, some of the microshutter on NIRSpec are stuck (they knew before launching). The MIRI has some image artifacts due manufacturing techniques.

The sensors were bespoke, even nowadays, we can't get sensors like the on on JSWT off the shelf. There isn't much use case for most of them outside space telescope. But we could do better with today technology.