You can do the same with the Steam installation files for games that do not have DRM - those that do wouldn’t appear in Itch or GOG in the first place.
You don’t own games generally. It’s always a license for software use. You may own the game, if you buy the company and the license is fully under its control.
Software is not a product. And there is no guarantee you will be able to run it forever, even if you made a copy of your entire setup. It’s especially the case with Windows, because it’s bound to a specific hardware that will break one day. Microsoft also cares less and less about gamers (see what they do with their operating system for consumers) and they have a way out with XBox. My bet is that Windows is not making money for Microsoft anymore and it will degrade more and more. Gabe knows it and has a strategy against it. If you’re a gamer and want have games on PC, use Linux and support the good cause.