You can't really ever have enough for a launch now adays... say you postponed for like 5 months and made more then you could ever need on release, one that's 5 months of delays that would be a huge advantage to competitors, also having too many they may not sell them all right away, also requires a larger initial investment before returns.
But really, that argument is reductio ad absurdum.
Really, Nvidia dropped the ball really really hard, even if they drop it a bit, it'll be still be better off than Nvidia, also, each day they aren't at market, there are more and more purchases of Nvidia products that will reduce their market size. So you can wait and build up what you can, but at some point, you just gotta shit or get off the pot. They must have decided that that was the date that would balance things out to a state that was acceptable to them.