why would a used game not count? You have setup a zero sum end game for attach rate.If I purchased 100 games, trade them all in for, say 20 games, my 'attach rate' is only 1/5th what it was before, and yet the attach rate of whoever buys those used  games does not go up. So games are dissappearing. The end game is if everyone trades games with everyone else, the attach rate would be zero - even with millions of games sold.

What you are measuring is 'number of games purchased brand new and still in your possession'.

That is not the 'attach rate' for the console. It's just a weird, random factoid. NOw if you also polled "number of games you bought new and traded awau" and "number of games you bought used" then you could get a feel for the 2nd hand PS3 game market. That might useful to publishers. Or not.