Since all us non-experts are piping in, I'll add my 2 cents. IIRC, the Hubble was designed to be serviced. I suspect that they designed the systems that were mostly likely to fail, or to be upgraded (such as some of the instruments) to be replaceable. But it wasn't designed to replace every conceivable system. You might design a computer so that the user can upgrade their RAM, their Hard-drive, and the video card, but not the backboard and the power supply. But eventually, one of the non-replaceable systems is going to need replacing. At that moment you say its dead, or you do the more difficult task of replacing the non-replaceable. And what is going to need replacing, and what isn't, is based on your best expert opinion when you design the thing, which of course, never exactly matches the reality 10 or 20 years down the road.

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