The thing is there is more people that (willingly and without "being forced to replace a broken console") bought 1 or 2 DS's than there is PS2 owners because I dont know a single person that still has a phat PS2 because all of the phats I ever saw in my entire life died (and I know like 45 or so people that had a Phat and now just have a slim because the phat died even my sisters phat died so did my PS2)
On the other hand I dont know a single person who had a Slim die. And the slim came out when the Phat already hit like 90-100 million units.
And when you think how big the percentage of broken Phats on ebay is and that 100% of all the people I know that had a phat "were forced" to buy a slim then its actually more than FAIR to assume that 20 million PS2s are replacements.
So there might be more PS2s but breaking certainly disqualifies for being KING. (same reason why PS3 already beat the 360 in terms of "meaningful" sales)
Its somewhat like the "shipped vs sold" difference















