I'm normally firmly against piracy, because I believe it to be morally equivalent to theft and I want to fund the artists making stuff I enjoy. But if Valve shreds my purchases when Gabe dies or retires, I will hoist the black flag on those games and not feel an ounce of guilt. As the saying says: if buying isn't owning, then piracy isn't stealing.
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.
They have shown its a wildly successful model. They would be very crazy if they changed it, and it would make them vulnerable to Epic and the Windows store. It's more likely that your OS/ hardware will change in a way that isn't supported by an old game.