Fleet commonality is an issue. But as the margins are much higher in the longhaul market compared to the by now barebone shorthaul market, it is still possible to disregard it for many decisions. If the longhaul low cost model actually works out, it will become an issue again. However, so far it hasn't really made much of a dent into that market and due to the much higher setup and upfront cost, as primera recently has discovered even on narrowbody aircraft, i doubt it will happen anytime soon. And of course, the crew/aircraft ratio on longhaul flights is by its very nature completely different. Where one can get away with 4 to 4,5 crews per aircraft on shorthaul, one will be hardpressed to use less than 7,5 (MFF) to 10 crews per aircraft on shorter-sector longhaul flights and well above that for ultra longhaul. Therefore the training cost per aircraft has to be higher per aircraft anyway, and more crews needed creating less of a pressure for common fleets and training cost savings across several fleets. Although an all-airbus fleet for example really cuts down on cross-fleet transition cost.