> Any code I do write myself eventually gets replaced with another open-source package. Even if the package is worse! When I've tried to talk to my manager, he's told me we have to think about how maintainable the project is.
There's a very strong argument to be made to never reinvent the wheel when building something new. However, if your product is only gluing together packages without doing any work, then the value it can produce is extremely limited. Where is the business logic? Certainly not inside Open Source packages.
There are lots of reasons to use open source packages for your code. Making the code more maintainable isn't one of them. Adding dependencies needs to be considered as a potential future risk, and integrations need to be carefully encapsulated to minimize the risk and allow easy switching later.
If anything, dependencies make your product more brittle, because you are now at the whim of Open Source contributors who may take the package in a direction you don't want, leave unpatched security holes, or abandon the project entirely.
Yes, you can fork the project if that happens, but then you're in the same place you were to start with, maintaining the functionality internally.. except now since no one on your team wrote it originally, you have no expertise on it.
In your situation I would be sorely tempted to look for a new job with a better engineering manager. And with more interesting business cases to build.