I think that this discussion has its roots in our preferences for the combat system in the game where we first encountered the Civ franchise. For those who began taking "just one more turn" in Civ3, Civ4, or earlier, it's expected that the player will create an army of 10 or more units in the ancient age, increasing to 20 or more in medieval and renaissance times. Stacks of 5, 10 are quite normal. Indeed, the AI will attack you with stacks of 10 or more in the BCE periods. The idea of taking a city with 3 ranged units (archers) and 2-3 melee units would be laughable in those games.
For those who began their Civ life with Civ5 and later, it's expected that armies are smaller, more highly promoted, and that units are conserved. Put a city under siege by surrounding it, breaking down the defenses over a few turns, and then conquer it. It would take scores of turns to produce (or faith-buy) an invasion force of 15 units or more. More than six ranged or siege units would have difficulty even getting close enough to take a shot at a city.
Over in the Civ4 forums, they asked about the largest stacks that players had (a) ever used, or (b) ever had used against them. A few players showed screenshots for case (a) of more than 100 units; AI players at Deity level often sent stacks of 50 more more. Yes, the AI knew / knows how to build a big army and how to invade the human player in Civ4/Civ3 and earlier. Those earlier games have *stack movement* shortcuts... surprise, surprise ... because moving > 20 units one-at-a-time is tedious. Optimal play is to *attack* one unit at a time, but dozens or scores of units can be moved with a single click. Those older games allowed the player to define a "rally point" so that newly produced units could move -- through and among cities and units -- without human intervention.
To me, it's all about what you expect to see, based on your first impressions. Based on the Steam data I've seen reported here, many more players got their start using 1UPT than unit stacking. Many long-term CivFanatics got their start using stacking.