I get your point, but a behemoth like Valve is so unlikely to be closing their doors in our lifetimes it's hardly worth discussing.
The real point here is that after spending thousands or tens of thousands on Steam, our next of kin or beneficiary will not get them once our lifetime ends because Steam doesn't sell games. They provide a license to access content.
Steam still suffers from the 'illusion of ownership' issue, and places that offer DRM free copies of titles are superior in this way. However it's plain for all to see that not many people care about this point. All the masses want is to play their games.
In that regard, Steam is king.
I do agree with you on corporate longevity in general, however I disagree when it comes specifically to Valve.
Unlike most companies that bring in billions in revenue, Valve is part of a comparatively small group of privately held companies. They don't have shareholders to appease, and they don't have a stock price to juice forever upward. I feel this factor alone puts them ahead of the herd, so to speak.
More than just being a non combatant in the stock market though, with so few employees compared to anyone else is a surefire way to weather even the longest storm in my opinion.
Microsoft themselves estimated a few years ago Valve's revenue to be 6.5 billion, which works out to 20 million per employee. Microsoft's gaming division brought in 16 billion in the same year, which is less than 1 million per employee. Even assuming Microsoft's profit per employee is a higher fraction of the revenue per employee compared to Valve, there's no chance its 20x higher.
Microsoft, Google, Apple, Exxon Mobil, whichever, I can absolutely see a future (that maybe I'll live to) where any of the humongous corporations die from providing worse products or service over time, or being knocked aside from the competition. In my view, primarily for the reasons I've written, I don't believe that will be Valve.
The main counterpoint I've been able to think up as I wrote this is whatever might happen when Gabe Newell dies. There's no doubt contingencies for this, and he's probably hand picked a few names he believes would carry on with his ideals. Though unless they've enacted quite iron clad bylaws or policies to prevent certain operational changes, the next leader of Valve could conceivably destroy the company as we know it. Barring anything serious I will outlive Newell, and so in this way, I can see Valve ceasing to exist in my lifetime.