That would be very distressing, but the important things (eg. Proton, Gamescope, OpenXR) they built would live on in their legacy. Plus, the PC games industry can survive just fine without Valve - but Valve can't survive without the support of PC gamers. Anyone succeeding Gabe would have to accept that, or squander what little value their shares possess.

Products like the Steam Deck or Steam Controller don't need any Valve software to play games. Valve knows a post-Steam world will exist one day, and they're fine with that. From a consumer standpoint, I respect that.