The direct transfer of power in tech is often to someone that will carry the torch. It's quite rare that a successor is picked that has been at the company for years, but wants to change practically everything about it. For that reason, I can see Gabe passing to a like-minded person that already knows that they are a succession candidate.

But ultimately none of us know Gabe, or what he plans to do. He may have a 100 step plan to secede power, or he might get to 65, say "that'll do" and just sell up and retire to a remote island somewhere. The plans might have been in place for years, or he might not want to consider Valve without him. Hell, he might not even think that Valve should exist without him. It's impossible to guess, so it's not worth worrying about...