While install base could limit the sales of GT5, there is one factor weighing in GT5's favor. Sony is on the ropes this gen. They don't have a plethora of third party exclusives to live on, so they've been forced to push their first party stuff as hard as possible, as their own titles are all they have left to differentiate themselves from Microsoft. This can be seen in the sales of their first party titles this gen.
Last gen, SCE saw five games that topped 3 million in sales:
Gran Turismo 3/4
Socom: U.S. Navy Seals
Jak & Daxter: The Precursor Legacy
God of War
This gen, these titles have already broken 3 million:
Resistance: Fall of Man
MotorStorm
Gran Turismo 5: Prologue
And these titles are on the verge of breaking 3 million:
Uncharted 1/2
LittleBigPlanet
These titles will likely break 3 million:
God of War III (pretty likely, imo)
Killzone 2 (it's inching along, and may cross 3 million one day)
And then we know GT5 will break 3 million.
That makes at least 7 SCE titles that'll have broken 3 million in the end, compared to only five titles last gen.
GT5 will likely also see increased marketing over its predecessor, much like Sony's other franchises this gen. This is their big kahuna for the EU, and they will make the most of it.







