"Time and usage will tell."
Time, usage and general acceptance as belonging to everybody sounds good to me.
I've never been sure of how copyrighting a song helps that process.
Right up to the 1980s Travellers in England were still making songs about their lives, if you pushed hard enough, you might just be able to find out who made it, it didn't seem particularly important to them who it was.
When you came across it later you would be told "that's a Traveller song" - a sign it had begun to take root.
It had nothing to do with style or subject, just how it was perceived by the people it involved.
The same applied to the hundreds of songs which were made in this part of rural Ireland, certainly throughout the period up to the mid 1950s.
Jim Carroll