> If he liked the GPLv3 he may have added the line back in
No, he couldn't have, because all the contributions received for the kernel were licensed GPLv2 without the "or later" clause, and cannot be relicensed to GPLv2 with the "or later" clause. Linux is GPLv2-only forever, essentially.
(It could relicense by stopping accepting contributions licensed without the "or later" clause, and either negotiating with all existing contributors to relicense or waiting for the copyrights to expire, but that's not likely to be practical. It certainly can't be relicensed on Linux or any other one actor's whim.)
No, he couldn't have, because all the contributions received for the kernel were licensed GPLv2 without the "or later" clause, and cannot be relicensed to GPLv2 with the "or later" clause. Linux is GPLv2-only forever, essentially.
(It could relicense by stopping accepting contributions licensed without the "or later" clause, and either negotiating with all existing contributors to relicense or waiting for the copyrights to expire, but that's not likely to be practical. It certainly can't be relicensed on Linux or any other one actor's whim.)