Someone might be willing to risk damages -- all the more so if they were actually limited to direct demonstrable loss rather than absurd... "presumed / possible / theoretical knock-on" loss amounts. But criminal prosecution?
Along with IP rights should come responsibilities, especially with items that have become part of common culture and dialog. If you don't fairly represent those rights to the culture/society, you should lose them.
In the patent world, IIRC, you can't simply use a patent to take a technology "off the market" / out of use altogether. (I'm no lawyer, and maybe I'm wrong, but isn't there language in the legislation mandating licensure under various scenarios?)
Neither should that apply to published works.
It already is criminal law, and has been for ages.
Copyright as way of trade - selling burnt DVDs, charging access to a torrent site, showing films without the correct permissions, etc - would all fit under the criminal bits of UK law. I'd be surprised if the US was any different.
Anyway, the question here isn't whether you get caught. The question is whether it's actually illegal if there is no copyright holder that anyone knows of, and one doesn't come forward with a legitimate claim. Will law enforcement charge you with a crime?
I'm not a lawyer and have no experience with law, but I do love free (and legally so) movies, so I'm genuinely curious about this.