Why will final fantasy XIII multiplatform make people sell their PS3's?

[Millennium]

Some, yes.

PS3 and 360 owners did a very strange thing this generation: they didn't wait for their favorite games to come out before buying the consoles. They treated their consoles as some sort of investment, biding themselves with 'lesser' games in the hope that their most anticipated game would make the console 'worth it.' FFXIII, MGS4, GTAIV, and Halo 3 were the most commonly cited examples of "investment games" for these consoles, and this is why three of these games failed to produce visible lasting boosts to the hardware sales post-launch: those boosts had been in effect since the very beginning. The fourth -FFXIII- has yet to be released.

This was a stupid thing to do, as the Wii owners -who generally acted more sensibly by waiting for their favorite games to be released before buying the console- could have told them. Things change pre-release sometimes, and this is a prime example of why. The PS3 investors lost: their console can play only two of The Big Four while its "main competitors" can play three.

All of this only seems rational if you're trying to quantify exactly much fun you're having, rather than just having fun. But that's how a console investor thinks.  Having fun isn't enough; you need to have MOAR fun, and to determine what this means you need marketers to tell you. This is the marketing model on which both the PS3 and 360 have based their entire business models for this generation, and now the PS3 is going to get burned.

It's over. Third place has been decided.