I don't know if this has been mentioned or discussed here, but I feel they should tie the science tree into simple upgrades of the unit without getting rid of the unit thru massive upgrades. Say with Rocketry, your Bombers get AA missles and it ups there defence against Fighters- your fighters would also get increased attack and bombardment bonuses with Air to Air missles or Air to Surface... this would also lend to the reality that first Generation units are normaly inferior to the last Generation version of the same unit.