Everyone's experience is different, mine, in this case, has been the opposite.
Library/package breakages caused by browser updates often take longer for the maintainers to release fixes for than I'd like to wait. I can fix myself and submit PRs, but this takes orders of magnitude more effort than fixing a similar thing in a library you wrote yourself; you can't be familiar with the internals of every external dependency, particularly as they're often generalised and therefore larger and more complex than if you write them yourself for specific functionality.
There's a trade-off of course, but I'd definitely consider updating libraries in response to browser update breakages a "con" when listing the pros and cons of using external deps.
That's how you evolve open source projects. If everyone were to simply copy paste snippets in their projects for fear of library rot, every library would rot.
> Fork, understand, fix, create PR
That 2nd step is more time-consuming for some dependencies than others. I'm a strong proponent for understanding how your dependencies work, but there are always limits to the depth of knowledge of the internals of every one.