What problem? If the copyright has expired on something, it has expired. If Peter Pan was to be freed, as he kind of has in the U.S. in a broken fashion, why can't someone who writes a Peter Pan book have it published, attributed, and protected?
Starting from the assumption that people have a moral right to impose copyright restrictions, then yes, we would want the opportunity to impose copyright restrictions ASAP, which would be as soon as previous restrictions had expired. If we place importance in freedom from restrictions, though, then there's no particular reason to rush to allow new restrictions, we'd want to ask whether there'd be a nett benefit in allowing new restrictions immediately.
The commercial release doesn't stop other people writing Peter Pan books; all it does is add to the growing collection of Peter Pan books. It adds rather than taking away.
A CC licence has little effect, other than allowing the owner to retain attribution and moral rights. A CC licence, being a copyright imposed on the work, is still more restrictive than letting the copyright (whatever the terms of the original distribution licence was) lapse. Restricting commercial licensing on derivatives restricts commercial interest which is of no benefit to anyone, least of all pirates who consume commercially produced content.
Two similar works may compete for the same audience. If there is a restricted derived work made from something, then this may reduce the likely audience for an unrestricted work, and thereby reduce the chance that someone decides to make one.
I'm thinking mainly of software, though, useful works for which free derivatives might well be developed, that undergo continual development, and for which usefulness depends on context (user base, documents, etc.). This is where 'embrace, extend and extinguish' is most relevant, I think.
BTW, there are a variety of CC licences with different terms. I'm thinking of the CC BY-SA.
By unrestricted, you actually mean fitting with your restrictions and expectations. Something that can only be achieved by the work being attributed to you as the owner and stipulating the conditions it and derivatives have to comply by.
I am thinking of unrestricted as being what the situation would be like without copyright law--i.e. freedom to use a work and its derivatives. Given that we do have copyright law, the only way this can be achieved is to prevent restrictions with 'restrictions'--i.e. you are 'restricted' from imposing restrictions. However this is certainly not the only possible way freedom from restrictions could be achieved--the other way would be to abolish copyright.