Because Old World expects combat to last multiple turns, allowing the player to overload a combat front with extra attacks (via stacking) was sapping away an important part of our unique combat system. Counter-attacks (and retreats) are meant to happen during the other player’s turn, and giving the player more opportunities to focus fire via stacking to kill a unit short-circuited that dynamic. 1UPT has multiple other advantages over stacking (including a cleaner UI, more cohesive fronts, and simpler combat rules), and we never looked back after re-implementing it.
In many ways, the Orders system, limited city sites, and one-unit-per-tile all need to be viewed as part of a single holistic system where each part buttresses the other. The enforced distances between city sites ensure that there is enough space between each city for a battle to be fought, instead of ICS-style cities inconveniently clogging up the battle lines. The Orders system prevents 1UPT traffic jams because the game allows a unit to make multiple moves with a single click and to covertly stack on another unit as long as it’s not the final destination. (Civs 5 & 6 do the same thing by allowing units to pass through each other, but because they are still limited to one move per turn, the feature is of limited use.) 1UPT, on the other hand, helps balance some of the extremities of the Orders system by making it more difficult to kill a unit via stacking. Each system cannot be viewed in a vacuum, and debates over the costs and benefits of each need to be made within the context of the game as a whole.