That gun was probably the only good thing in that movie.
Digital distribution is in its infancy, many companies will come and go before it can be considered a truly reliable source of content.
Look at what Amazon is doing with the Kindle--when they lose the copyright to distribute an e-book, they remove it from your device without your knowledge. Music sites are constantly going up and down, losing untold amounts of consumer investments.
In my view, Steam is the only current content delivery service I can see existing for the long run.
It seems to have stable distribution relationships with the major publishers. Valve is well positioned as a quasi-independent firm which, now with Steam, is no longer beholden to a publisher and is therefore unlikely to be purchased by one (even though they have an agreement with Activision for publishing, they don't need them), which would them upset the agreements with others.
It looks to be in it for the long haul.
Chances are, D2D will be purchased by someone who has no interest in supporting the content which they are currently obliged to supply to you. Whether or not you will still have access to your purchased material after that is no better than a coinflip's odds.