The reason Linux is still GPLv2 is because they didn't include "or later version" is the license text, which basically prevents them from upgrading to GPLv3. If they wanted to change this they need to track down every contributor to ask for permission.
Technically, yes. But the net effect has been that it's become very popular with companies that depend on the unfixed loopholes. You are right that the reason switching for Linux never happened is also that it would require permission from the copyright holders.
But if I recall correctly, Linus Torvalds was biased against this in any case (probably for the reasons I outlined). Either way, the license has served Linux well.
I would even say that without the GPL attached to it (with linking exception) the Linux kernel wouldn't have been where is was now.
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