I can understand the reason for giving Archers and other Ranged units 2 range. It serves to differentiate these units from the other classes, adding to the tactical complexity of the game. Ranged units are flexible in that they can attack up to 20 different tiles around themselves, and they also have the advantage of being able to attack without receiving damage in return. They are vulnerable if attacked in melee, and to fast-moving units. Terrain and elevation becomes a bigger factor, as Ranged units can either greatly benefit from it, or be greatly hindered due to cover and sight limitations. Rather than just smashing armies into each other, you have to pay careful attention to positioning and movement.
Of course, this would work a lot better if the AI had any tactical ability at all. As it stands, the tactical depth of the combat system just serves to make combat extremely easy for the human player.
You have to pay careful attention to positioning and movement in tactical table top battles, too, of which I played 1000s with miniatures in a long ago era. And in all of those, massed (non skirmishing) ranged units had an approximation of their proper, historical use of being able to deliver a first strike against units that are about to melee attack them or who are trying to move past them or to prompt the enemy to charge them or retreat by making standing in place untenable.
So while I get what you're saying, and agree this is a positive objective, I think paying attention to positioning and movement will be there every bit as much with all range 1 units as it is currently. Some of the tactics may differ (no more Archer rush, please!!) but the interest will be just as great.
If, instead, we bump Slingers and Machine Guns up to range 2, I'd be okay with that, too. Slingers had an effective range at least equal to, if not greater than, simple bows, and machine guns can outrange crossbowmen. You can look that last one up (actually, you can look both of them up)
. And at least there'd be some internal consistency, which has to help when programming the AI.
And I agree wholeheartedly that part of the game design challenge should be to create a system that is simple enough for the AI to play well, but challenging (EDIT: and interesting!) for the player to master. The potential for that is there in Civ 6, with the rock-paper-scissors aspect of the unit classes combined with terrain effects and flanking bonuses. But then it's thrown off by the weird balance choices between the unit classes and complicated by unit class promotions that require you to scrutinize each unit of yours and the enemy (no obvious clues as to the promotions they have). The end result is a military system (and overall game) that is hard for newbies to pick up and understand, but easy to master (in the sense that a fairly small investment of time will push most players past the AI's capabilities).