I would be inclined to think there's less risk of the team leaving than of individual contractors leaving. They've found a gig, are working with their friends (or at least people they've chosen to work with) and are committed to each other as well as the project to stay on.
This is solid. The team is not going to leave as a whole as much as any business would abandon a healthy (or maybe not so healthy) customer relationship. I also think that people will leave the team for the same reason they'd leave a healthy employment relationship: startup opportunity, education, family, retirement, etc.
If the whole team does decide to move on, it's likely that the set of circumstances that would push a "packaged team" like this away would probably cause mass attrition with normal employees as well. Just today a friend of mine told me the 2 layers of management between him and the CEO quit on the same day, and he's feeling like he wants to quit now as well. So goes the pack.
