This assumes, that the practices/methods used to create a working library are suitable for solving the problems. They might be ill-advised and include tons of workarounds for bad design decisions. Too often following the advice of never reinventing anything (and possibly doing better), is how we ended up with stacking broken stuff on top of other broken stuff, limiting us at every turn with leaking abstractions and bad designs.

It is very possible to have a clean implementation with good design choices overtake an established in time, enabling more extensibility, modularity and maintainability.

An example: People still way over-use regexes for all kinds of stuff. Even in code editors people for syntax recognition, where people really should know better.