>Linus can't close the kernel.

That's not how it works. GPL only prevents old versions from becoming closed source. If Linus added code to the kernel which required a $100k license to redistribute then people could no longer freely distribute the code of the kernel. People could not freely distribute compile kernels because they would need that license. GPL doesn't magically make all licensing issues go away. He could also make a required kernel module that was not GPL licensed that Linux could require to operate.

>Except there is no device with free drivers that it supports.

Having a working system providing competitive value to others is much more important.

>They just refuse to support Librem or Pinephone without a good reason.

The good reason is that those devices can't provide industry standard security.