Or, like the Hubble, when it starts out broken.
Exactly. This much money being spend on a one shot deal that absolutely has to have every single component work perfectly and completely free from any possible intervention once the booster lights off on the launch pad.
It might make more sense to assemble it in orbit, say at the ISS, then launch it into place from there. But the ISS is no more useful for a mission like that than it is for any other defined purpose I can think of other than to give the Russians a place to hang out in low earth orbit and break their own endurance records.
For as much money as they are spending on a one shot, highly risky mission like Webb, I would expect to see a full time manned observatory at L2, but that is such a fantasy it doesn’t merit serious discussion here....