I anticipate the idea behind not having such a button is to prompt the player to consider replacing lost troops with a combination that is different, in whole or in part, to what it was most recently. That said, your post got to me thinking how such a function could be applied. I think a dialog box prompting a player to replace lost troops -- with an option to have the game do so automatically to its most recent composition -- would be of benefit. For players like myself, the ability to check a box to disable such future prompts and instead be able to access such an ability through an on-screen menu would be welcomed further.