Don't forget that those archers are 50 miles tall, making a few hundred mile shot not such a big deal. Seriously though, worrying about misalignments of scale between real life and any of the civ games is kind of a ridiculous undertaking. Bring the game clock into it and things can get really out of hand.
A stacking change like that would defeat a lot of the purpose of 1upt, which is to allow for tactical placement of units. If you can park a defensive unit on top of your artillery then that artillery unit no longer really has a vulnerability to melee attack, which is the weakness that justifies such a powerful ranged attacker. If you were going to allow some stacking like that the penalty would have to be quite a bit rougher I think. Something like, the melee unit defends, but at the strength of the artillery, or greatly reduced strength so that the melee unit is mostly just cannon fodder to prevent a unit from sneaking into your formation and sniping your artillery. Also, if the artillery fired that would have to prevent stacking for that turn or you could just move a defender off, fire, then move that defender back on.
@ the OP: Artillery units are definitely incredibly powerful defenders in a city, they're my choice every time it matters. They also take down enemy cities with surprising ease though. The key to using them is to analyze the terrain and zones of control of your defending units as well as knowing where the enemy units are coming from so that they are not vulnerable to counter attack. It's not trivial, but the payoff is big since you can take a high strength city without putting your own units in much danger at all, allowing them to get the promotions that make for really powerful units down the line (if you hit dynamite with 3-4 range/double attack cannons ready to go, that's the game).