I think it's more that this argument can be continued ad infinitum. Yes the JWST had significant cost overruns, but in the design phase these types of projects necessarily are looked at as how to best utilize a fixed size grant of money. At the end of the day the decision got made to make a better single telescope than to make two simpler telescopes.

I wouldn't be surprised though if we start to see clever design proposals coming down the pipeline, like several cheaper telescopes, swarm designs for giant radio telescope arrays, and even amateur designed and operated space telescopes. Remember that the JWST started it's design phase back in 1996, and the economics of space launches have changed considerably since then.