more minions coming out ath enemy, they should get the xp for killing them, it's plain and simple. nothing is wrong with some simple risk and reward
its a very minor risk, but this also allows for the infamous comeback - say you are getting cocky and want to buy some priests to push and end early, well this is one of the few mechanics that matter because it is when the game starts to swing, and by nerfing xp, it takes away that swing value, making comebacks even harder
an upgrade can still be an upgrade if it involves risk, of course none of you seem to want that, you want priests then victory. i'd like priests to be able to reward the purchaser, but if bought at wrong time harm them - every upgrade can do that - buy xp upgrade too late, buy gold upgrade and not get enoguh out to cover it, buy death penalty upgrade but you dont die
there are risks to your purchase, minor ones, but now minions have none. thats good to some, not to me