Commonality has some good arguments but so does aircraft capability and price.
AB is the best example of this. The B737NG and A320 family have so very little in it when it comes to operating costs that is all to do with ownership costs and financing that dictates how most races are run.
Long term 737 operator, AB were playing the game with Seattle and wanted the price lower than Boeing were prepared to go. In the matter of a few weeks AB did a deal for the first 60 320 series aircraft.
After being in service for over a year now AB has gone back to Boeing for replacement of the older 738’s and additional growth…… why?
It is a well known fact that the best bit of the NG is the Wing (and the extra row of seats) and the best bit of the Bus is the cabin. In the real world of bums on seats AB could not operate the Bus from some of the network destinations at the same pax load of the NG due to airfield limitations that the NG wing gives better performance.
The other reason is delivery slots, the Baby Bus has been selling like hot cakes and AB’s options have been limited due to production capacity. Boeing has the capacity.
Then there is the DBA factor….. AB has A319/A320 & B73G/B738….. DBA has 733 & F100 which have already been slated for replacement by 73G. However AB & DBA have found the lower cost and capacity F100 very very useful in route development and as a replacement when capacity is not required.
So for a growing organisation AB have used the F100 fleet effectively being able to match demand, capacity and cost where larger types may not have been viable.
In a similar manner AF use Cityjet with the “four oil leaks connected by and electrical problem” and a lower cost base for many routes where they could use the 318 to places such as LCY (when approved) given that they have every type of baby bus and you would expect the commonality argument to prevail.