Let's think of it this way. Let's say you're a painter and all your life you've been painting and selling those paintings for a decent price. You're successful, your work is well sought after.
Now, after a few years, you decide to stop painting and you no longer want to sell any of the paintings you haven't sold yet. Do you not have every right to do that? Surely, even though there's a demand for your paintings, doesn't mean you absolutely have to sell them. Even more, just because somebody wants them, does that justify them coming into your home and stealing them from you because "well, it's not as if he's losing money"?
Obviously this isn't the same, but I think LucasArts has every right to say "We no longer want to sell these games, but we still want to protect our copyright.
It's also a legal issue, if LucasArts suddenly said "Okay, fine, if you want to download it for free, download it for free, it's not as if we're selling it anymore" then they lose a portion of their rights to the Monkey Island property (technically, in short it'd set down a precedent that any lawyer could take advantage of should someone wish to use that property without buying the rights). Monkey Island as a trademark is valuable to them, not as a game, but a franchise. Yes, I sincerely doubt they're going to make a Monkey Island 5, but at the same time, that trademark is one they've held onto most vehemently, so they don't really want others to get their hands on it very easily.
It's for this reason that the law you've proposed about software becoming abandonware after 5 years of not being available to buy is something that would never come to pass. By law today copyright lasts about 50 years, and even then a person or company has the right to pay a nominal fee so they can hold copyright for another 50 years. If your proposed law came to pass, people and companies would lose control of their own trade marks simply because they chose not to sell it.
Again, it's really down to the copyright holder and what they wish to do. Just because you want something enough doesn't mean you should be allowed to get it no matter what.