Clearly it would be ahead of 360 and perhaps even challenging the Wii - although I think that would also require different games.
The other issue PS3 has, apparent from price due to BR technology and loss of BC, is that during its first few years it has been host to mainly teen/adult games and most of the core developers seem to be working on new IP that is darker/more mature.
Nothing wrong with this, but a huge element of PS2 (and PS1) sales - was that you bought it for your kids and they played R&C, Sly Cooper, Sonic, Crash, etc. and let you read the papers in peace.
Therefore the Wii has stepped right into this role, and it being bought by parents the world over for their kids.
But sure, it would be selling a lot better. With hindsight I feel Sony thought they could just ignore Nintendo, and focus on beating MS early on, while letting PS2 slowly die and waiting for PS3 to pick up steam on the back of developers flocking to it.
However, assuming this is a correct if very simplified guess of their core strategy, things didn't work out like that because:
1) the Wii succeeded on its own due to its controls/games being lapped up - and you'll notice that while Wii new games seem weak now early on Mario Galaxy, Zelda, Mario Kart, etc. were all in place to boost the console early on. PS3 lack of compelling content for kids early on and steep price simply made the choice easy as a parent. When I bought our Wii it was a companion to our PS2 and shared space on the same SD TV
2) 360 caught a massive boost with Gears. Up until then it only had Halo, but two shooting powerhouses, an easy to use SDK, and a decent install base in the wake of the Halo/Gears one-two made it an obvious choice for developers to aim shooting/adult games at
3) PS3 turned out to be a bitch to develop for initially, with poor SDKs. As a result it suffered a year of 'inferior' games which hurt the console more than I believe is often recognised. Certainly it boosted the 360 as the 'better console' when I'd argue the truth was the 360 was getting the 'better developed' games.
4) BR had a long and tough battle with HD - with MS trying its best to make sure each format lost (Oh, Toshiba, how naive you were, or simply too keen to have MS 'on your side')
As a result Wii roared away with the family/casual market that had (and still) prevents the 360 getting to the kind of sales numbers we're seeing for Wii right now, while the 360 had enough install, games and kudos to hold of the PS3 for a long time, particularly in US.
Then, when the PS3 was catching up (as it had been for a while) MS were in a great position to lower the price in time for December while Sony were still trapped by more expensive hardware costs.
Ah, its been a funny old battle so far, hasn't it? And no sign of ending anytime soon either - despite all the PS3 doomed threads its clearly not and there's lot's of fun still to come IMHO.
Try to be reasonable... its easier than you think...
