May I offer an alternative reason why Valve did not make Portal 3? It's because when you rehash the same game, people get sick of it. Sometimes a product needs to stand on it's own. Why harm the Portal franchise by being to quick to release, release, release?

Seems like hollywood is already on board with this. What do they need Ghostbusters to have a 100 year copyright on when they could just release a new Ghostbusters every 30-40 years like they're pretty much already doing.