Expiring copyright is just that: you can copy the distributed result (binaries) as you want. It does not entail open sourcing any more than it would force the studios to release their raw footage and CGI files.
Oh another thing: very important:: a short term of copyright like a few years would be the biggest boon to Hollywood ever as they could simply sit and wait for works to drop into the public domain before turning around and producing them without paying the creators a penny. There would be tons of creators strung along via a studio option - just long enough till the work dropped into the public domain. It would harder than ever for individuals to profit from their creative work and easier than ever for Hollywood to make money off of it.
What creators, exactly, are you talking about? If you mean screenwriters, they could simply sign a simple contract ("You shall not produce a movie based on the work without authorization") before showing it to the studios. Why would they need copyright?