That argument is a slippery slope towards asserting there should be no quality control on Wikipedia; any debatable WP article could be defended by saying "tag it and hope for sourcing".

What people keep forgetting in these arguments is that WP isn't pronouncing a final judgement on the topic; they're judging the article itself. If you believe sourcing could be found for a topic, and a better article could be written, go write it. There's a zillion "proven" articles in WP that need maintaining; why argue about the ones people can't bother to bring up to some minimal standard of quality?