...Gamers expectations always increase. What new consoles do is raise these expectations by increasing the performance of the games, whether it's just graphics, or more the gameplay (think shattered horizon).
If the PS3 was never launched, Sony would not be in control of the HD media format, and 360 sales would be much larger then they are today as gamers would have played the likes of GRAW and Oblivion and realised that those games are simply superior to there PS2 counterparts.
Saying PS2 graphics are good enough is utter bull. What you fail to realise is that graphics are a fundamental part to getting good gameplay. Take the GT series. This game tries to be an imitation of real life , therefore better graphics help with immersion. This is true for any game trying to be realistic. Then take FPS' - better graphics allow most to work better due to the nature of the genre. If graphics are poor, you cannot distinguish between a guy and a bush due to draw distance. Yet if graphics are better, you can. I doubt that an intended part of the game is to make things look indistinguishable at a distance due to low res textures, poor lighting etc.
You are also utterly ignoring the PC market. Whether you like to admit it or not, this market impacts consoles and therefore has to be considered when debating a consoles success, hypothetical or not.
ATI and Nvidia would have hardly stopped making GPU's and pushing graphical boundaries because the PS2 was still popular. Graphics on all gaming platforms tend to move together, and because of the point above about graphics can mean better gameplay, gamers would demand better than ps2 graphics.
Plus you also ignore all the other parts of consoles. The problem with the PS2 was it was maxed out. Games like San Andreas showed that the PS2 was struggling to keep up with devs ideas and this resulted in problems like slow loading trees, large barren maps etc.
You need to realise that gamers expectations are always on the increase. The PS2 was not able to fulfil this role, nor would have been able to, for a full 10 years, because simply put, the 360 could offer better games. Whether you like to admit it or not, the truth of the matter is that better hardware allows games to be better.
Plus you ignore the PS2's nonexistant online capabilities, wired pads, tiny memory cards etc.
To be honest I think you need to play for a year on the 360 and PS3, then hit up your PS2 and realise how much better this gen is to last.