One problem with "operational range" is that if player can extend range by plopping down a settler somewhere, what good would it really do?
About the only solution I've found to that question would be to give settlers a much shorter leash than military units. Players could not take their unit stacks with settlers to some places. However, all this means is that there would have to be a longer chain of "bases" between the player and his target. What value is added to the game by doing this? All it really seems to add is a bunch of hoops to jump through, useless complications. It might effectively render ancient-era conquest of the entire world impossible, but would that be FUN? Who would bother with much ancient era military? Players would become too secure in the knowledge that they are safe from attack. Especially in Civ4 multiplayer this would probably harm the game balance.
Even in single player, though, we need to think it through with care. It might be fun to prevent complete world conquest in the ancient age, but the game would HAVE TO allow for conquering a neighbor, and if range is loose enough to allow for that, players could "island hop" their way to a complete victory. So what good have we done with range limits?
The original Master of Orion has limited range. It has up sides, but one can lose his entire fleet if the "supply lines" represented by range limits are cut off behind him. This actually makes the SoD phenomenon about TEN TIMES WORSE than it is in Civ.

Since one cannot afford to risk significant forces on deep penetration strikes, this dictates that player reach out only to attack from secure bases. This takes every option off the table but for the massive SoD. Is that really where you want Civ to go?
Then there's the water problem. What if continents are separated by more water than the range allows units to travel? What about archipelago maps with lots of small islands? If we allow an exception on the water, for ships to travel any distance, or we allow ships to act as supply posts, that destroys the purpose of messing with range limits in the first place. Doesn't it?
About the only kind of range limits that might bring up-sides without fatal flaws would be range limits that function early but are loosened over time and disappear somewhere along the way. At best, we're talking about taking ancient-era world conquest off the table. This won't help SoD at all.
In a game like Civ, more is better. More money, more units, more cities, more power. What use trying to change the nature of the game? If more is better, then SoD wins, because SoD is the active pursuit of "more military than the other guy".
Any meaningful fix for SoD, which managed to undermine it as the winning military strategy, would change Civ into some other kind of game and probably kill off the entire franchise. After all, the winning move will either be to accumulate the most, or else there will be some arbitrary point where accumulating more starts to hurt instead of help. The former can vary according to the map. Different maps allow different maximums. The latter would be a single flavor that does not vary from game to game, a single formulaic solution that comes out identical no matter what the map is like. This "cure" would be far worse than the disease. What use throwing out the baby with the bath water?
SoD is here to stay.
- Sirian