I think they made GW go obsolete at the point of dynamite because otherwise the builder of GW could park artillery a tile back from his borders and shoot into any adjacent border with relative impunity, even without any units in front guarding the artillery. The only units that would be able to come thru and attack on the first turn would be horse-units, which at that point aren't going to necessarily do enough damage to kill an artillery unit before getting blasted by the GW-owner's artillery first.
It's the same reason France's UA goes obsolete at Steam Power: if France could get 2 extra culture per city all game long, then they could rob cultural victory from people by simply using quick moving modern units and their late-game UU to quickly conquer and puppet a handful of cities. Part of the reason France's UA is balanced in the early game is that it's a relative risk to try and go out and rush neighbors, and even if successful in the early game, there's no way France can cover enough distance quickly enough to make their UA overpowered. But once they have tanks, planes, battleships, paratroopers, and all other sorts of fast-moving units, their UA would become a little broken because they'd easily have a shot at cultural victories just by conquering and puppeting cities, which might be a bit too much for small cultural civs that played for cultural all along to overcome.
