That's assuming PS3/360 did not expand their reach beyond PS2/Xbox though, which I don't think is a safe assumption to make.
Where there is audience overlap, there is competition.
If the number of Ps3 + 360 sold collectively is more as or less the Ps2 + OG Xbox it means than the numeber of new customers who entered the market is more or less equal to number of those who left.
You have competition when customers percive that two or more goods are both viable choices to satisfy the same need. Canonical example: mobile phones. Phones weren't competing with laptops and computers unitil they beacame a viable tool to browse the internet - so two viable options, same need.
The customers who wanted to play FPS / online games didn't even percieve the Wii as viable alternative. Same goes for those who bought the Wii for Wiisports. The Wii didn't have any direct competitor unitil the Kinect came out.
That's not to say the Wii didn't compete with X360 and Ps3 at all, but the competiton was marginal. The Wii was percieved as tool to satisfy a different need than PS3/X360, and that also explains why many PS3/X360 players also had a Wii in their houses.