By reducing damage by 70%, it takes 3.33x as many attacks to strike for 100% damage. I'm making some generalizations, but if you're attacking from max range, it takes 3x as much time to contribute the same amount of damage.
That is assuming your unit can attack without ever receiving any damage in return
. On the current version I would argue that the Range promotion increases not just the safety of an attack but also the average damage per turn because the unit with extra range essentially never has to be rotated out or healed. Without extra range your units will need to enter the attack range of enemy units and take damage in order to deal damage. Obviously your units then cannot attack every turn because they would eventually die. What percentage of turns your units without extra range actually spend attacking is highly situational but for units that do have extra range it's very close to 100%. So the average damage per turn when attacking with extra range would not be 70% lower but I'd estimate something like 30-60%.
Also consider that any excess damage on a target is wasted. It's not that rare for a unit to barely survive an attack with ~5 HP. And if an attack with -70% damage can finish that unit off it's just as good as a regular attack. I don't think it would be that hard to maximize situations like this since you can simply save your comparatively weaker attacks for after you've used your regular attacks to compensate for any potential bad damage rolls.
Yes, you get a shaving of damage when your unit can't reach its normal targets, but I feel like that's not even close to the power of other T4 promos.
I don't feel this way. In VP the jump from field guns to artillery isn't as extreme as the jump from cannons to artillery in vanilla but the increase in power from the extra range is still very high. By contrast the jump from cannons to field guns is not nearly as noticeable even though they get Indirect Fire for free. Other T4 Promotions like Firing Doctrine and Infiltrators only give situational bonuses to CS which translate to a modest damage increase. Logistics gives a noticeable increase in damage per turn but only if your unit doesn't move or pillage that turn (and since it requires either Barrage III or Field III I think you also have to take the worse promotion path for ranged/siege units first).
At -70% damage I would consider Range as one
of the options since it would allow me to effectively field 0.3 additional units and minimize risk while something like Firing Doctrine would simply give me a straight damage increase but with comparatively greater risk to the unit and more downtime.