There another variable in the system that prevents bigger airlines in expanding using alternate fleet groups:
The fleet commonality feature has a bit of a flaw. Instead of it being a reward - lower operating cost when you own many aircraft of the common fleet type - it is implemented strictly as a penalty. Penalty for operating more than 3 fleet types. This penalty is implemented exponentially, based on number of aircraft you own.
While you own small number of aircraft, the penalty is small, barely perceptible. But once you are between 100 and 200 aircraft, the penalty starts to be seriously felt. And when you get to 300, 400, even 1000 aircraft, this penalty will bankrupt your airline.
This is the reason big and successful airlines concentrate their purchases in 2-3 most popular fleet types.