5m doesn't seem like a stretch by any measure as long as the game continues to be issued as a budget platinum title.
Considering the impact of the game, I don't see it going out of production for the life of the PS3.
Personally, I'd still like the opportunity to buy a re-vamped MGS4 along the lines of Substance and Subsistence. Full install would be a welcome addition plus additional play modes, playable Raiden stages (using gameplay elements from MGS Rising), etc.