The big "test" there for Steam will be when "Gabe goes away". It's gonna happen sooner or later.
But we'll see. I hope it doesn't come to that. That said, I'm trying to change my purchase habits over to GOG because even if Gabe's successor doesn't screw over the Steam customers, eventually someone will. With GOG there's no possibility of the games I pay for being taken away from me.
That being said, I believe that Gabe is taking his "succession planning" seriously, so I'd be fairly optimistic for the next decade at least.
He probably already has a will set up that details how ownership should be transferred.
[1]: https://www.tomshardware.com/video-games/pc-gaming/trump-adm...