> Each release, large parts of the kernel get rewritten anyway.
That's not really true, and if code is being modified rather than completely replaced it is probably a derivative work of the original code (meaning the license is still GPLv2). The only practical way to make sure the license change is legitimate is to get permission from every single copyright holder.
> Lots of kernel code is copyrighted by companies now.
And some of those companies may have gone bankrupt or been dissolved which puts you in a similar situation to a contributor who has died, except it may be even harder to figure out who exactly owns the copyrights.
> If they are dead, their copyrights are held by someone else now who can be asked.
Even if you figure out who owns the copyrights, do you really think you'll be able to convince every single bereaved spouse, sibling, or child to "relicense your dearly departed [father/mother/wife/husband/brother/sister]'s contributions to Linux to GPLv3"? I envy your optimism.
So yes, it will probably take too much time and effort and give too little benefit to be practical, but impossible? No.
> Even if they're dead.
If they are dead, their copyrights are held by someone else now who can be asked.