Like you said, imagine trying to download a game like MGS4. I wouldn't want to know how long that would take. And then there's the load it would put on the servers trying to accommodate all the people trying to download the game. I don't know if PSN would have enough bandwidth for such a large number of simultaneous downloads. Granted that they could update the servers and what have you, but would they do that?
Bottom line is that I hope they keep the blockbuster games on discs. As much as I do enjoy getting games from PSN, I would not want to have downloading the "big titles" as my only option.
Even though most of the games I have on my ps3 are from PSN (all small games), there are definitely certain titles that I would want to have a physical copy of, the "big titles" as you put it. Even if they had the option of either buying or downloading certain games, I'd probably lean more towards buying it (assuming pricing, game version, availability, etc... was all the same).