This is true, however without copyright I think many other things would change making the GPL a lot less necessary. For example, no one could prevent you giving your friend a copy of a program, so the market would change a whole lot. Basically, the thing about BSD that GPL doesn't like is that BSD permits proprietary software being created. Without copyright, there isn't really proprietary software any more, and less of a reason to keep source secret.

(Most of this thinking comes pre-web-applications, that does change the parameters a bit.)

Yes, and mathematicians were supported financially by universities, by the patronage of the wealthy, or were independently wealthy. Wherever knowledge production and creative work occur, someone is paying for it.