While I agree MGS is not going to sell like Halo or GTA (for a Sony exclusive, that's GT's job), I think the numbers above are a little misleading.
When MGS came out, there were not 100m PS1's, and when MGS2 came out, there were not 120 million PS2's
Fair enough, but when people pick up a system, do they not often pick up previously released games that appeal to them? Hence games having legs? And shouldn't your point be somewhat muted by the fact that MGS3 had more PS2s out when it came out, yet sold the wort of the three? Obviously there are other reasons for that, but MGS3 is just as well thought of as the other two in most circles, at least as far as I know. I think it's the best of the three.
My consoles and the fates they suffered:
Atari 7800 (Sold), Intellivision (Thrown out), Gameboy (Lost), Super Nintendo (Stolen), Super Nintendo (2nd copy) (Thrown out by mother), Nintendo 64 (Still own), Super Nintendo (3rd copy) (Still own), Wii (Sold)
A more detailed history appears on my profile.