AT&T was not in the Android game because they had the iPhone. Verizon was in the Android game because they didn't have the iPhone. The carriers will go where the consumer goes. If Verizon's customers stop buying Android in favor of the iPhone it's of no consequence to Verizon's.

IMHO the reasons Android sells are:

1. It's not the iPhone. A minority of Apple haters rail against the notion of a walled garden;

2. US specifically: their chosen carrier didn't have it. One thing about the US I haven't seen elsewhere is that family plans are fairly common;

3. Android competes in markets the iPhone doesn't, specifically low end.

You'll note that I didn't mention cost as a factor, other than buying lower end phones (which is obviously a significant market). To reiterate this point: all the top end phones in the US have $199 up front and this isn't a significant factor over 2 years of service so, if this is a factor, it ranks far below carrier choice (IMHO).

Personally I think the Verizon iPhone will probably add a net 10M iPhone users in the first half at least.