It's not cost-effective if you standardize on something ridiculously expensive, so that you end up doing counter-insurgency CAS or homeland air defense with something that's designed to go up against S-400s.
Look at it this way: Suppose you need a family wagon and something to haul the boat, but you want something that's fun to drive. You can buy a Porsche Cayenne GTS but for the same money you can buy a Ford Explorer and an MX-5. What's cost-effective?
Aircraft support & logistics, too, have become much more efficient with computer networks, supply-chain-management tools and worldwide delivery. Result: savings from commonality may not be as important as they used to be.