Yes, it is uncertain, but Microsoft really isn't under much threat from a look-alike OS that most people will consider outdated by that time. Only geeks and diehard anti-vista freaks are likely to adopt it, and among the latter, just those that absolutely have to have some level of Windows compatability for their apps or development. But for developers, most will throw in the towel and just move to whatever platform their potential customers adopt, and use the best tools that they can find there.
If they, by some miraculous chance, actually become a threat to Microsoft, then it could threaten them and offer to buy them out at the same time. In other words, the offer they can't refuse. But that is the business model. It has nothing to do with the merits of the project itself.
If you look around, you will see several MS-DOS workalikes still exist, some still under development. These are tolerated because Microsoft does not attempt to sell MS-DOS any more, so there is no motivation to fight them. And if you look, you can find that QBASIC and QBX are both posted online. These are no threat to sales of Visual BASIC or Visual Studio. So if there is no perceived threat from an XP workalike, there is little chance that matter will go to court.
There is increasing pressure for companies that walk away from products to relinquish their legal and intellectual rights. And it is a minefield for both sides, because if Microsoft or another major player takes on a challenger, they are inviting the courts to write new law, in the absence of legislation. They may not want to go there, because if the ruling goes against them, that could have major consequences to the industry in the future. Microsoft does not want the added expense of having to go back and provide indefinate support to products it no longer sales just to protect any rights it still claims to them. They are apparently happy enough to leave the whole issue unresolved and a determent to others that might be tempted to clone outdated products and sell them under their own labels.
You also have at risk whole industries that refurbish products as to their legal position. What if we could not retread tires anymore? How about people who restore antique cars, or repair old pianos, or restore homes? There are a lot of grounds for challenging the claim that manufacturers have to long term ownership rights in the aftermarket.
So it is a fair question, but I don't think we are going to see it fought out over the issue of ReactOS.