If I grew up under a rock, somehow became a self taught musician, and ended up authoring a song that had recognizable components from Happy Birthday, then even still the author of Happy Birthday, having established that melody so successfully in the public zeitgeist, reasonably should benefit.

This is so common the recording industry itself has established rules for sampling and licensing and covers and what not. Are there some folks out there abusing the system, for sure. But overall its goal is to maximize the value produced by the recording industry, which very much includes the people who 'got their first' who built foundations for future artists. To me, this all seems basically reasonable.