> I was on a team last year that was in their second round of developing their own in-house UI toolkit

Every team eventually tries this and fails to some degree, incurring costs along the way.

Also it's a lot like Akin's 39th law of spacecraft design:

https://spacecraft.ssl.umd.edu/akins_laws.html

But replace "launch vehicle" with "in-house UI toolkit".

It baffles me that people time and time again make the judgement that their several months of work are somehow more worth(in terms of effect) than literal man-years put into a decent UI library.