You are forgetting about existing phones upgrading. Admittedly, it can be a long process for the hardware vendors to certify ICS for their phones, but 6 months is probably a reasonable time frame.

One thing that has changed about the relationship between Google and the hardware vendors is that Google now has it as a core objective to get the vendors onto the latest platform. In the past, I don't think that it was a high priority on Google's part.