Addendum: just to clarify, installing 3rd-party library dependencies is a good idea. There are many good reasons to do so. This just isn't one of them.
I've encountered ~100% custom codebases that are unmaintainable, and I've encountered ~100% custom codebases that are extremely maintainable, even for a new person joining a team. And we're not talking about leftPad-esque libs; entire frameworks made in-house.
It's rare, and there are reasons to avoid it (locking yourself into your own tech, not benefiting from community innovation, not being "on trend" with common dev conventions). But ease of maintenance is not one of those reasons. If it's hard to maintain, that's on you.
