[Aengeuad]

It could go several ways but I can't see a judge wanting to entertain complaints that a new GPL version isn't to your liking if the changes are minor, especially so since it's always going to remain valid in GPLv3.

If the FSF goes rogue and changes the GPL to be incredibly restrictive (i.e., allowing proprietary redistribution, and I realise this can be considered permissive..) it might be possible to get it to be ruled invalid defaulting to the more permissive licence, especially if you have deep pockets, or if the FSF change the licence to be ridiculously permissive like 0BSD then it's not going to be legal in countries like Germany, either way any major change is likely to result in an international enforcement nightmare.

[ufo]

If GPLv4 were more restrictive, people would still be able to use the software under the terms of gplv2 or gplv3.

The scenario where FSF goes "rogue" would be if gplv4 became more permissive, e.g. if it stopped being a copyleft licence.