Well, here my very own opinion: They are wrong. They are looking at the North American market and predict, that their 1923th Star Wars installment will sell better that a new graphic adventure game. If they turned their eyes towards Europe, they would see that latest adventure games like Secret Files Tunguska, So Blonde or Perry Rhodan sold a lot better than any Star Wars game. I used to play Rebel Assault or Tie Fighter vs. X Wing, but these games have grown... boring.
The fact is that at least in Europe adventure games are again growing in popularity. They will never be a challenge to shooters, RTS, or MMORPG, but a lot more popular than Star Wars games - over the years.
The other fact is, indeed, that Lucas Arts could not make another adventure game even if they wanted. The guys that were in charge in the better days now work for different companies that still do adventure gaming.
So, my conclusion is. For God's sake, bury your hope of another Monkey Island installment. You would really dislike it if it's been made by LA, and the chances that they would sell their rights to someone else are naughty, it is more likely that hell freezes over.
What I meant to say is, why is lucasarts still holding onto the rights? Do they have anything planned for Monkey Island? Right now, it looks like nothing's being planned, so why hold onto the rights?
Well, some people do the same with real estates, they buy old buildings in town an let them rot, hoping that some day prices will go up. Lucas Arts does that with the Monkey Island rights. I hate this kind of speculation and it should be disallowed, but its existence is a fact.
Because in the future, they *might* do something. Or not. You have to view it this way: Those assets have a *potential* worth. If they retain them, there is a chance (no matter how small it might be) that they might earn money with it again in the future. OTOH, if they release the rights, the chance is 0. So, from an economical point of view, it makes absolute sense for them to keep it. It's that simple.
You are right at this point, bu economically it would make sense for them to sell licenses to those who are willing to make a new game. They keep their original rights and the license holder keeps the whole risk of a failure. If the game sells off, LA will earn with every copy sold. If not, they would at least haven't lost a thing. And they still have the rights to make another MI game at their will.