You can request to take over the existing package if it’s broken (-git packages tend to pull the latest version, so don’t fix it if it isn’t broken).
You are not expected to maintain it forever. However you should make it a -git package if you do not intend to update it on every upstream release, so it stays up to date without changes to the PKGBUILD.
It’s only self evident if you accept your own morality is subjective. There are people who think morality is defined by some invisible friend who is right about absolutly everything.