There's some good thoughts here, but I think that in large part, the developers being this sloppy with external libraries are going to be equally sloppy with in-house code. Why would someone bring in Bootstrap even though it wasn't effectively addressing the use cases? Why was it left in place after so much of it had been discarded? If they are "programming with the delete button" on Bootstrap and keeping that clean, do you really want this same team manager some homebrew system? Will they leave unmaintained, semi-dead code paths all over the place just waiting to be exploited, or cause some sort of other issue?

Misuse of libraries is a "brown M&M" to me, it's a symptom that I need to take a hard look at how the team is behaving, and solving that will resolve the library problem and a whole host of other issues too.

J.Ja