[REGL]*hedja*{CS} you have very bad sense, if someone chooses to do an airlift of that size at cost of other bonuses, it means that the bonus they will attempt to break is way more important than the ones they may leave at risk. What's more, it's not so standard that they 'll have to leave any bonuses at risk. It is possible, but not standard.
1st of all, why would your game have stacks of 2000, you are using a map too large to be good for LD or simply with awful bonus values...
2nd if all of Player A's moveable armies are now on that 1 border, then it will be easy for Player B to expand elsewhere with the extra 1500, and use it to break the other bonsues. You can't assume that 1 border is so important that Player A would have all troop and Player B only 25%, is must not be that important or Player B is a noob (do you happen to be player B?)