Because the Start Screen IS the new start menu. Microsoft has put forth a TON of effort on building it for touch and mouse. Setting that aside, it would be a HIGHLY difficult sell for Windows 7 users to say to them, "We have a new version of Windows called Windows 8. It is literally exactly like Windows 7, but more faster. You want to buy?"
A difficult sell to you maybe.
I'd be willing to buy a better performing, lower resource using and more secure OS.
Businesses would have no problems buying it either (assuming that the price is right).
"Windows 8."
"Better performance!"
"Improved security!"
"Runs on current hardware!"
"No retraining required!"
It practically sells itself.
I agree that they won't change course now..
MS (Sinofsky) have invested so much time and effort telling everybody that they know best, that they won't make any changes now.
Wikipedia - Denial of Responsibility.
Denial - Wikipedia, the free encyclopedia