The penalty kills airlines. AKA fee paying players, in their droves.
No problem with badly managed/out competed airlines going down, but when your commonality costs rocket insanely overnight due to flying one extra plane then we have a very silly situation.
We can retrain pilots, why not the other staff? Why not have a transition fleet designation? Say, we have two or three years whereby a fourth fleet can be flown and we take it that staff are receiving training and manufacturer assistance to run the new type? A premium could still be paid, just not in excess of $20m a month!
Or, reduce the fleet separation. 737NG and A320's have a nice transition into their modern counterparts. This forces players into this game style. The B717/MD90 meld is a good step in the right direction, but by the 2010's they still have nowhere to go and have to introduce a new type.
It's a good game, with an impressive detail until we come to this issue, whereby the realism falls apart completely. It's not a balancing mechanic, a change in this would see a far greater variety of types flown at all ends of the spectrum, which makes for a fuller, funner game.
BA currently fly 7 fleet types in the AS world, with an 8th on order.