Hmm, I think the second time the gyros needed replacing, and the gyros were the limiting factor on the telescope's lifetime, I'd have installed a redundant set.
Well, except if system wasn't designed to have a redundant set of gyros, it could be that some complexity and risks may be involved. For example, unable to write that without changing some board, reprogramming, etc.
All such satellites (incl. ISS) have 4 gyros while you only need 3, geometrically. That‘s your redundancy. You don‘t add stuff just for the fun of it but you do cost-benefit trade-offs that include e.g. weight (goes directly into launch costs) and design lifetime. And gyros need to be heavy because physics (i.e. you need counter overall mass of spacecraft).
