You forgot DEAD in your description of the grandmothers
There are several problems with this. Even if Open Source did,which I do not believe it does in any way that cannot be immediately rectified(For example the FAT modules might be considered that now that MS won the patent on that FileSystem), MS has no clear cut strategy to be able to sue open source. They could sue the companies supporting open source, but that is a mixed bag, as the companies themselves do not claim to own Linux. All they do is sell servers with Linux on it, or provide support for it.
They could sue the companies using Linux, of course we all know how well that turned out for SCO, and what a brilliant image that brought them(And some still suspect MS of having more than a hand in that maneuver as well of course). Difference here is MS has the money to do with SCO could not, carry on a pointless and losing battle until the other company is bankrupt. It is unfortunatly a common tactic these days. But then what happens if the big names all band together to support it, would all of them together have enough money to continue the fight? Probably.
They could try to sue the developers...
Except the majority of the developers are not something that can be sued easily, and of course there is the question of which one do you sue. You sue one that had nothing to do with the code in question, does it do much good?
So their best course of action will be to sue those they want to give them money.
Now that being aside, with the exception of very recent IP patents(Good grief I hate that system), I don't believe they have a good case to base it off of anyways. Even in the recent IP of the FAT FS and such, I am not sure they can prove a loss of sales based off that anymore. Windows is not suited to embedded devices, which is the primary place where FAT is used these days. What this is doing though is trying to scare folks into using MS. Gotta love fear tactics. In other venues that might almost be called other names, somewhat akin to 1920s style mob activity. Using fear to control the actions of others.