You're forgetting something, it's not the same design.
The sticks are different, the triggers are different, the weight is different. They did try to "fix" it, but in all the worst ways.
The erganomics are the same, are they not?
I can't say I measured my PS2 pad to see if the joysticks and buttons were in the same place, before I sold the PS2, but it certainly felt the same to me.
The weight I'd always put down to simply because the weights of the dualshock mechanism were missing, which seems to be right, as my DS3 weighs more. The Joystick erganomics seem the same to me too, though the deadzone on the Sixaxis is definitely worse than the DS3. I honestly can't remember what the deadzones on my PS2 pads were like, but I'd imagine better than the Sixaxis.
As for the triggers? They could be better! But, they don't take long to get used to either. When I first got my PS3, and got R:FOM, I remember thinking "hello, what's going on here" with the triggers... but by level 2, I thought nothing more of it.
Thing is, I'm not arguing that the PS3 pad is better than the XBOX pad, they're both different, and both good. I do like the little concave bit on the top of my XBOX pad, but the PS3 has a grippy cover on there, so doesn't need it. They're different, they both work, I think they're both good designs.
V1N.