My understanding is that even if the man at the top wanted to, Linux is copyright all it's contributors so all would have to agree to a relicense. This would be significantly more effort than it's worth given there is no shortage of contributors happy to use GPLv2.
It's legally complicated. In the past, Eben Moglen has said that he thought it would probably be possible under some sort of collective work theory. But certainly others disagree and Torvalds was not a particular fan of how GPLv3 skirted close to usage restrictions anyway.
There’s also the problem that many of the older contributors probably aren’t reachable at the email address attached to the commit. They may even be dead in which case you’d need to find who the copyright transferred to (next of kin? Did the will give it someplace else?), and get their permission.
They certainly could mandate that all new contributions are licensed as GPL2+, not the GPL2-only a lot of it is under. Then, after an eternity, when every bit is now GPL2+ (either by relicensing or rewriting), mandate all new commits are GPL3+. Then the only way to use it would be to rip out the GPL3+ parts (if you want GPL2 terms) or to use the GPL3.
The theory I guess is that, even absent a contributor license agreement, contributors to the kernel are effectively contributing to a collective work and that the project as a whole could make changes even if individual contributors did not agree.
