Let me play devil's advocate for a second...
What if someone files a patent and had all the intent in the world to make that invention become true, but due to funding issues, could not after a certain period of time? Then some big company claims "look, that guy isn't doing anything with his patent!" and shows they're willing to pool resources to making that invention real.
Unless you all mean that at that point, the patent is public domain. I guess we should stick the caveat of "you may not make money off your patent".