\r\n\r\nHowever if Bob chooses to abide by GPL v2 in his distribution and it came to him GPL v2 or later, he can choose to license the result GPL v2 or later because that license violates no copyrights and grants all of the rights to the recipient that GPL v2 requires him to grant. (Any recipient has the choice of choosing GPL v2, which grants them the necessary freedoms.)
Um, no. Here's where you don't seem to "get it":
\r\n\r\nIf Bob does takes some action which requires him to accept and abide by the terms of the GPL, then Bob has to choose one and only one version of the GPL to accept and abide by. This is the same as any other dual- or multi-licensing scheme: you don't receive the program under all those licenses at once, but instead choose one and abide by it.
\r\n\r\nAnd it is because of this that Bob's options in licensing the program to third parties will be limited; since it's the situation we've been working with previously, let's assume that Bob chooses to accept GPLv2, and that GPLv3 imposes additional restrictions on use which make it incompatible with v2. Now, Bob can modify and distribute under GPLv2 in that case, but he cannot make the "any later version" offer. That would mean offering to license under GPLv3 and since we've postulated that GPLv3 imposes additional restrictions, and since imposing additional restrictions on his licensees would violate the GPLv2 which Bob has accepted, he can't make the "any later version" offer without violating GPLv2.
\r\n\r\nAgain, the moral of this story is that a GPLv2-incompatible GPLv3 would be the mother of all headaches.