I don't think it's fair to push this all on the package manager. If a dev
introduces new dependencies that need packaging, it should be their responsibility to make sure this is done.
Indeed. I am willing to build *some* packages (and submit them to Debian), but that willingness is decidedly not infinite.
The first question that somebody who proposes to add a new dependency should ask themselves is this: is there a way to accomplish the task at hand without adding a new dependency? If not, can the task at hand be accomplished using packages that are already in Debian stable? If not, does the dependency at least have an active third-party ITP (intent to package) in the Debian bug tracker?