As one who said I anticipated more variety on PS3, it was because of Sony's first-party titles rather than 3rd party exclusivity. Plus the argument wasn't so much more variety, but guarenteed variety. Buying XB360 early on, you had (and still have) no idea what range of titles its going to see. You could be sure of Katamari on PS3, but was Katamari destined for XB360? That was very much an unknown. PS3 was set to inherit PS1 and PS2's diversity, so it was a safe bet for those wanting that diversity of software. XB360 may still showcase a massive variety, but it wasn't a sure-thing back then, and importantly, it's diversity isn't likely to be any better than PS3's because 3rd parties are likely to go cross-platform as everyone was saying.
And they were thinking against the consensus because of a biased view. There was no reason for these games to stay exclusive (ignoring GTA which is already cross-platform, and RE which not only is cross-platform but also was announced for next-gen consoles)