GOG is no different, you're still renting licenses and GOG still has the right to revoke your license, effectively making your "offline installer" no different from a game downloaded from myabandonware or a similar website.
Supporting developers is a weak argument considering that GOG's claim to fame is that they're selling old games where the development studio no longer exists or has been bought out by a corporate entity like EA.
Revoking my license isn't a big deal? I paid real money for the game.
The offline installer is about as much of a guarantee of anything as a pirated ISO is.
Also Steam doesn't apply any DRM unless developers add it, so backing up your Steam library folder to an external drive should be fine for your personal preservation at a platform level.
Transferable licenses create a second hand market which keeps prices in check, which of course publishers don't like at all.