why SC can't currently let Heroes under upgrade to compete in war
Apart from loss of revenue from those willing to gem their heroes (i suspect their biggest draw) the heroes in normal are under construction and cannot have a reneration time when they get beat in war.
If they were to entertain doing this, not only will they be giving up a revenue stream but expend more money to accomodate this feature.
In short this having heroes which are upgrading in war is a bad idea unless you wish pay for playing the game.