I don't think a system where copying is always permissible under particular copyleft terms, where creators can neither restrict that copying nor grant further/o

I don't think a system where copying is always permissible under particular copyleft terms, where creators can neither restrict that copying nor grant further/other license, is appropriately referred to as a system of "copyright" - no one possesses a unique right to control copying. Note that I'm not speaking to the advisability of such a system, just pointing out that there's not actually a theoretical dependency as I would use the terms. If we're being loose enough with the term "copyright" that it applies to any possible limitation of copying, then of course copyleft depends on it because we've just defined it to include copyleft - that is not interesting.