I truly, sincerely wish we could return to an age where folks were happy expressing a (valid!) opinion. Nowadays, everything has to be tortured into seeming more "objective", and therefore something that is disliked, and / or not understood, is automatically called "arbitrary" instead.There isn't some underlying design rationale behind arbitrarily deleting some of the player's units at certain points throughout the game.
If it were arbitrary, you wouldn't be able to save them by stacking them inside Army Commanders. You wouldn't get a free Commander. They wouldn't regenerate a while after death. And yet, all of these things happen. It's very much intentional, which is the complete and direct opposite of "arbitrary".