There are various legal arguments I can think of, that would essentially trash the GPL, and a company with 8 figures to spend on lawyers could make very convincing (I don't agree with these, but a judge and jury could easily):
1. It's unclear whom the actual copyright holder or holders of Linux are, and who has standing to sue on their behalf.
2. Since Linux is open-source, free-as-in-beer, etc, it's public-domain.
3. Linux is a derivative work not entitled to copyright protection.
And so on. Again, when one side might have orders of magnitude more budget for lawyers, this kind of stuff will be very convincing. (Many corps will pony up money to protect Linux, but they'll generally be on the opposite side when it comes to the GPL).