This is helped a lot by the fact that's it's quite hard [EDIT: = expensive] to place an upgraded telescope somewhere. Hubble is still used despite it's tech being a few decades old.
My point it is, we have so few space telescopes that even having time on an "outdated" one is still pretty valuable.
EDIT: Maybe a better example, it's not much use for a first-world person to use a ten year old smartphone, since current smartphones are much more capable and they're comparatively cheap. Planes, on the other hand, are used for decades, since simply buying a new one is very expensive. This is despite it's tech being far superior.
It's the massive capital expenditure on the telescope and its infrastructure that is the significant limiting factor on the pace of putting newer and better telescopes in place.
Same thing in this case: instead of visiting Webb, they could build a new Webb without all the origami, and assemble it by hand in orbit for a tiny fraction of the cost, and end up with two of them. Then, maybe put different instruments on the second one.