One problem with the casualty choice concept is a sub overwhelm. If the enemy has enough subs, and you lose all your destroyers first, and any subs survive the first rounds of combat, the attacking player can retain those. In a normal casualty choice, the last destroyer would not be lost until most or all of the enemy subs were destroyed.
This puts the defender at a disadvantage and alters the dynamics of the offense vs the defense. It forces japan and the atlantic allies to modify the way they defend because they have to set up to make their own overwhelming attack before the sub stack can attack.
Of course the defending ships on the sea will be able to fight back against the subs, if they survive the intial attack. But the planes wont–and the retention of the fighters + 1 DD in the later rounds of combat is one of the most powerful defensive combinations. If you’re out in the open sea, they can also exploit this dynamic to crash all your fighters, having first striked your carriers out from underneath you.
There aren’t any easy fixes to this problem, because its not only an issue with losing lower value units first. I want to lose all my defensive subs, then all my defensive dd except 1, then move on to losing better stuff.
I suppose there could be a checkbox or default to “lose all dd 2nd (after defending subs), except 1, until I’ve lost all my air, or the enemy has lost all their subs”
As others have pointed out, the changes made to facilitate async play may at first seem to have a small effect, and they would have a different effect on differerent versions of the game because the rules for subs and destroyers change. But the order of casualties issue obviously causes a major departure from the normal game dynamics, in all versions.