That argument worked before AQP. The DC-9, MD-88, and 717 all have the same type rating, yet I still had to take up lots of sim time to fly them. The extra 2 sim sessions are a pretty minor cost for an airline. Most airlines would rather pay more money for a plane that wasn't routinely getting grounded. Airlines buy 737 for 2 reasons, they are cheap and Airbus can't make enough A321s to fill the demand. And the only reason 737s are cheap is because everyone knows Boeing is hurting for orders.