How do you suppose that you download a blu ray sized game? I've got an 18mbit internet connection, and downloading a 25+ GB game would take quite some time. Lets also not forget that a lot of people have caps on the amount they can download. Download 4 games in a month, and your ISP may very well cut you off. Also, you would need a large hard drive to hold all of your games. I'd imagine 1TB would probably be necessary for a lot of gamers. I own ~20 PS3 games, and I'm not exactly hard core. I've just had the console since it was released, and I get ~5 games each year. (Side rant, I typically buy the hot games a year after they come out. Preferably when they release the next sequel. It's amazing how many games I've bought for $19.99 that were released a year before for $59.99. Delayed gratification really saves a lot of money. I suppose if I was playing more online gaming, I might be more inclined to get the hottest games).
I see digital downloads as the future of gaming, but I don't think that Sony or Microsoft will make it the ONLY method of getting games until the majority of people buy their games digital by choice.