I know we can debate the balance of the bronzeworking technology, but that's beside your point of "either include every resource or include none at all".
Commander Bello said:
So, in general any assumed missing of copper harms you in the early game, when your economy is weak and you are assumed to not being able to build a considerable force in time to get some.
There are LIGHTYEARS of difference between copper and would-be saltpeter.
Without copper...
- You'd be able to build all kinds of wonders without worrying about defence.
- You'd be able to discover multiple religions without worrying about defence.
- You'd be able to settle all kinds of cities without worrying about defence.
By adding copper into the mix, you shift the weight away from purely peaceful strategies. Players have to either discover archers and catapults pretty quick, or prioritize unlocking copper. With copper unlocked, they need to make smart decisions about their city locations.
Copper prevents early peace from being too powerful. And players have LOTS of control over their access to copper.
The key difference is this is the early game, when there's already an extremely level playing field. You can theoretically find copper whenever you want, and do so peacefully or aggressively.
Saltpeter would do almost the exact opposite, because most of the cities would be founded and land already assigned. Saltpeter would not be in control of the player, but wherever it is lucky to appear. This would assign a random advantage to a player -- which is NOT strategic in the least. Moreover, your probability of gaining this huge random advantage would go up the more tiles you control. Meaning the person who spent their time making huge land grabs in the middle ages would be guaranteed to win... while the person trying to build those wonders gets the shaft.
Adding Saltpeter would make peace too WEAK. Players would have VERY little control over their access to Saltpeter
Civilization 4 is a success because they killed snowball effects like this. It used to be that if you discovered gunpowder first AND had it, you'd have won the game, rendering everything after 1000 AD moot. Now the game is actually competitive for much longer, with small nations able to hold out and make a real attempt at victory.
If you prefer the game where you open up a small advantage in 2000 BC which you ride all the way to a predictable victory, there's still Civ 3.