This would be hugely distortive of game mechanics, like Low Luck alread is, but this would impact especially hard on the small battles, where Low Luck is not as far away from regular dice as in the big stacks (likely a main aim, as you reference the single bomber against single transport case).
A simple example is that, with these rules, you would easily choose to send 2 infantries to attack 1 infantry, as this would be 100% success. Hence, sending 2 infantries instead of 1 infantry and 1 artillery becomes much better a choice, heavily distorting units balance.
Just making you notice that you are not just reducing the randomness of the game, once you get to the combat resolution phase, but changing the game to some other game, that will play out differently, modifying your strategy in purchase and movement, as well.
On top of this, assuming such rolls will remain separate, this would increase the relative impact of increased randomness when having separate attacks/defences in the battle, like AA, submarines, naval bombardment, or when air cannot hit submarines but other units can.
Finally, you would not cover all cases anyways, as, for example, sending 2 fighters to fight 1 battleship will remain about as risky, with all the possible outcomes, ranging from the battleship sunk with all fighters surviving to both fighters destroyed and the battleship surviving.