They don’t have to provide a way to install the games in perpetuity, but I’m pretty sure the ToS don’t provide a way for them to stop you from keeping or running a DRM free copy you’ve downloaded.
So sure, the ToS says you don’t own the game, but unlike ubisoft that puts that non-ownership into practice, GOG goes out of their way to make that legal non-ownership utterly meaningless. If you have a copy of the game, then you have a copy of the game.