There were DARPA-funded projects back in 2004 to try to avoid doing it like this, while it was being built. They were funding investigations into assembling the satellite on orbit, e.g. send up the segments one at a time and dock them together on orbit. Those projects were cancelled in W. Bush's second term after he pushed for manned missions again. To this day, I still think in space assembly would have been cheaper and possibly lower risk.
JWST and most scientific instruments are extreme one offs that take a long time to build and don’t have backup parts. So if anything trying to dock multiple parts in orbit would have likely increased costs and risks.
There is a very small risk of a launchpad failure, but sending up multiple launches only increases those risks. Further, building a second one would have been much cheaper though still very expensive.
The basic technology of robotic space construction, though, could have far reaching benefits in its own right. So while it may not be strictly better for this one mission, it might still pay put dividends in the long run.
