Earlier quoted context omitted.
I bet when current management leaves, as usual happens in these cases, those that worship Valve and Steam will see yet another big corporation, this if it doesn't eventually get acquired.
What management? Gabe doesn't have any direct involvement in the running of the company at this point, in spite of being the majority owner. It's an open question what happens if he leaves or dies but it doesn't seem likely that he'd do anything to intentionally sabotage the company, like sell it off. It's more likely that ownership will transfer to a trusted party or to the employees.
At this point I think Valve is more likely to be disrupted from an outside influence rather than within, I think a lot of their actions are about keeping the ship steady. If someone does come along as an upstart in a new market as valve did with digital distribution versus retail/physical, the old is unlikely to be instantly irrelevant and obsoleted, and even then you get new markets like mobile appearing over their lifespan where both are healthy side-by-side.