Well we should really go further.
Streamings Capacity vs cost is massive. For $8 a month I can stream unlimited hours of content . That is the cost of a value bluray.
Streaming a game a la Gaikai isn't the same experience as playing locally (plus only needs a thin client, so is a different model) and you'll be paying more than $8 a month as you need to fund the servers and internet distribution.
If you mean digital distribution, that's had its pros and cons repeated to excess. It's the future, but not yet. Hence the recent B3D poll showed unequivocally that gamers want both download and disc options.
In engineering, you'll see Ashby plots showing materials weighted against two variables.
http://www-materials.eng.cam.ac.uk/mpsite/interactive_charts/strength-density/NS6Chart.html
You'd then pick one that had the best balance, meeting minimum requirements and trying to maximise secondary requirements within budget.
We have the same thing with game distribution, dealing with several parameters.
What you do is pick one variable to argue against a choice (like speed), and then switch variable to argue another option (price) without seeing the balanced whole of all the variables. discs sit in a strong mid-range position, being cheap and capacious and fairly fast. Flash tops the speed plots but bottom outs the cost. DD rates higher than discs for cost effectiveness (assuming someone already has an internet plan) but is far worse for speed.
When you 'plot' all the parameters, you see discs occupy a very solid central position, rating very highly on the primary requirements (cost, capacity) and well on everything else. There's no parameter where discs are dire. That's why they are used and will continue to be used until technological changes in the other options shift their plot positions to make them more balanced.
