Sigh...older units do not suddenly disappear when newer ones become available. Relative range means NOTHING in this game, it is entirely about different unit types.
It doesn't mean nothing to this game.
When designing a game like Civ, you have to look at and research (almost) every type of military variations that exist. But of course you cannot put everything in. And others you tweak to make them into the unit type you're looking for, for the sake of the game making sense.
However, those decisions also depend on the 'range' of the era you're looking at. If you properly scale a slinger to a tank, the tank would have to have a range of, at minimum, 10 tiles. Rocket artilleries hit at least ten times that, so let's just give them a range of 100 tiles, right? That would break the entire game into nonsense.
Instead, the concept is to 'scale down' the ranges every two eras or so. This results in the situation whereby compared to a Rocket artillery a tank is pretty much in melee range.
This is why Machine Guns were initially given a one-tile range; the maximum range of heavy machine gunfire can even be about twice that of a tank. That had to be changed to serve gameplay reasons...
So it's not like they never consider these factors. It's a mix of both. Sometimes realism gives way to make the game better. Other times, realism adds to the richness of the game.
And when you don't have to worry about slingers if you have tanks, this disparity isn't that big of a deal as far as the game is concerned
