Is it really that big a loss? LucasArts hasn't done that well lately.
Personally, I think Disney's done quite well over the years. They partnered with Square Enix to give us Kingdom Hearts (a series that was at its best when Disney was most involved with it). They bought up Marvel and we got The Avengers.
Sure, they drive their own properties into the ground, but they tend to be respectful of other people's ideas.
But they're not a gaming company. They don't really have the know how or experience to turn LucasArts around and make it a good company again. Just letting it go to free up the IPs so that they can turn them over to other companies seems like a much better idea for Disney.
Had they purchased LucasArts at its pinnacle, I'm sure they'd keep it around and working. But they've inherited a company that's really fallen into a downward slump, with little to recommend it and with no real idea how to fix it. I don't really mourn its loss.