> since you can choose forever, everyone can pick
You cant choose forever. As soon as someone touches it with v3, it's v3 and you can't get it back.
The most common and easiest case where that happens if that someone integrates some other GPL code into the GPL project.
The contributor didn't realize that GPL(2) and GPL(3) are two very different things. The code integrated / copy-pasted from elsewhere was GPL3. If not caught and removed immediately, the presence of ANY GPL3 code, just one line, requires that the entire project be released _only_ as GPL3. It can no longer be used under GPL2.
The reason for that is that the new contribution that is GPL3 licensed wasn't licensed under GPL2. Since that bit isn't licensed under 2, the whole package can't be distributed under 2.
The wording in GPL3 is unclear in such a way that it could pose a very significant risk to people who aren't even remotely involved in open source at all. Whether that wording is merely stupid or devious, who knows. The problem was pointed out before the license was approved, and the wording wasn't changed, so perhaps Stallman actually did intend to leave the threat there, while claiming the threat didn't exist.