Total war games have an auto resolve button if do not want to fight a battle.
Auto resolve is *never* an option.
It basically falls into two categories: the auto resolve can fight a battle as well or better than the player, or the auto battle does worse than the player.
If the first is true, why would anyone play out a battle? The AI can do it as well as you can, you're just wasting time (and for multiplayer, wasting EVERYONE'S time). Sure, it might be fun the first few times as eye candy, but that's not enough to justify using the resources to make it. The majority of the player base won't use it, so why bother?
If the AI does worse than the player, the player is pretty much forced into playing out battles. Sure you can skip it, but when you lose half your fleet from being lazy you'll never skip one again. It's an illusion of choice, not a real option.
So which of those two cases do you want "optional" auto resolve to fall into? It's going to fall into one of them, regardless of anyone's intentions otherwise.