This has strong vibes of “If only Linus Torvalds had charged for Linux, he would have been a rich man today.”. It does not work that way.

Not at all similar. Linus explicitly made his software free. Romero didn't _intentionally_ exclude the copyright notice, and had no explicit intention of making it free.

It’s not “ironic”, it’s completely expected. If it was only an old black-and-white movie, still subject to copyright, today the movie would be a historical footnote at best.

"completely expected" is quite a stretch. Simply making it public domain wouldn't be enough. It still has to be a good movie. I'm sure there are countless other public domain black-and-white movies that no one has ever heard of.