The baseline take on what happened with Webb is that too many low-readiness technologies were included when the mission was approved and went into “Phase A” which means that development starts.

The entire astrophysics community suffered as a result of the ensuing suckout of resources. Lessons have been learned because a lot of careers were impacted.

There’s several successors to Webb on the horizon, and current thinking is to mature the technologies needed before such missions enter Phase A and is in effect committed to.

For a concrete reference on this, see the (very large) National Academies survey, which charts the course for NASA astrophysics over the next 10 years:

https://www.nationalacademies.org/our-work/decadal-survey-on...

A decent gloss on the above report is:

https://www.aip.org/fyi/2021/astro2020-decadal-survey-arrive...

which lays out the tech maturation plan under the “ Flagship mission maturation program” heading.