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....
I'd suggest that you're right about assembly at the ISS. It could be done right there in ISS's hanger module. ;) Well if the ISS doesn't have a hanger module, we need to build one!
Surely the smart folks with the Webb Space Telescope program can figure a way to modularize the telescope to the point where it can be launched using existing cargo rockets.
We also need those space tugs we were promised back in the 50's to move telescopes/satellites/other instruments to higher orbits and back to the ISS for maintenance.
It would make more sense to put 1/10 as much resource into very large adaptive optics ground based instruments.