I don't think that's necessary. Had MS waited for HD-DVD, even just 6 months, it's very likely they wouldn't be in first place in NA right now. I mean, we only have one data point to go on but it appears reasonable, no?
In the US the 360 leads by nearly 11 million. More than a bit more than the 2.9 million they sold in the first year. So that's certainly questionable.
Worldwide I have no doubt the ps3 would be ahead if the 360 had delayed.