We had a library written by a former employee who was a prolific producer of code. He insisted we needed it and spent over a year developing it in company time.

The library was a masterpiece of what if driven development. It was about 50k LoC, and it had 300k LoC of dependencies. It was a nightmare to modify. And no one wanted to take over maintenance so people would submit PRs to the former employee when they did modify it.

I wanted to change something in the library to support a large migration I was in charge of. When I went digging it turned out that we were barely using any of the features in the 2 years since he’d finished it. I replaced the 50k LoC library and 300k LoC of dependencies with 300 lines in less time than it would have taken me to modify the library (a few days).