Ah... the PS2. I don't think I can ever remember a console that's dominated its generation in quite the same way. I'm not just talking about unit sales (though its figures there and its lead over the Xbox and Gamecube were impressive enough), but rather about the sheer scale of the influence it exercised over gaming in general.
Back in the PS2's generation, if you were developing a console game, then unless you were being given bags and bags of money by MS or Nintendo, you had no choice but to make the PS2 your primary target. It didn't matter that it had underpowered hardware that was known for pain a pain in the arse to develop for. The Xbox and the Cube were optional. The PC (which was on a back-foot for most of that console cycle) was even more optional. The PS2 was where you had to be to get the sales. It had games from every genre represented; and often the best titles in their respective genres were for the PS2.
In many ways, it wasn't a particularly brilliant console. Its UI was butt-ugly. Cross-platform ports tended to look like a dog next to their Xbox and Cube versions (though the latter were admittedly quite uncommon). The memory cards for savegames were tiny, expensive and prone to data corruption. But it had the games, so if you were at all passionate about console gaming, you had to own one.
The funny thing is that, despite its hardware being completely obsolete, I've often felt Sony sent it to the back burner (via the PS3 launch) too soon. Both the console and its games were still selling well when the PS3 launched, with the 360 having failed to take much wind out of its sales. I do wonder what would have happened if Sony had held back the PS3 for 6-9 months, to work out some of the oddities in the hardware, let the launch price fall, get a stronger launch-lineup and maybe get proper back-compatibility into the hardware as a standard across the world. As it is, when the PS3 launched, it was too expensive for most and still suffering fierce competition from its own predecessor (some of the PS2's best games launched after the PS3, such as Personas 3 and 4). Certainly, for the first 18 months I owned my imported US 60 gig model, it spent far more time running PS2 titles than PS3 ones.
Nothing in the 360/PS3/Wii console generation has come close to replicating the PS2's dominance. The Wii got a big installed sales base early (which later stagnated, with the result that its lead, while still there, is much eroded), but never even came close to converting that into PS2-style dominance of games development. The 360 and the PS3 have more or less run neck and neck; if I remember, the 360 has a small worldwide installed base lead despite its Japan deficit, but the gap between the two isn't much more than a rounding error. And if you're developing a game these days, then unless you are being given large amounts of cash by a console manufacturer, you need to target the 360, PS3 and PC (the latter is very much back in the game), while giving consideration to the idea of a Wii-U port or a scaled down Wii version.
I wonder whether, to an extent, the PS2's dominance wasn't linked to Sony's ability to lock down what were, at the time, some of the biggest and most important franchises in the world to its console; Final Fantasy, Gran Turismo and Metal Gear Solid. Those were really the names that started shifting consoles (after what was actually a slightly lacklustre launch). These days, of course, all of the really big name franchises are cross-platform (and almost all Western, rather than Japanese). A couple of exceptions; the Nintendo first party games (not everybody's cup of tea), Forza (the 360's superior reflection of Gran Turismo) and the Halo/Gears vs Resistance/Killzone shooter pairings (where the games are essentially interchangable). But increasingly, it's cross-platform that dominates the charts (particularly when it features angry men with thick necks shouting "OSCAR MIKE" every 5 seconds).
PS. Another Final Fantasy XI expansion? My word. I stopped playing that years ago and didn't realise it was still going. It feels a bit like a relic from another world now; easy to forget it was probably the world's most successful MMO until World of Warcraft launched.