Good point. Diplomacy trait only keeps its edge up until your competetion reaches Alphabet. That being said, if one of your non-Diplo opponents were to make a b-line to Alphabet, then the lead would be greatly diminished. However, diplo civilizations working in concert would almost certainly be the civs founding religions and building the early wonders. But you are right, the price a diplomatic civ has to pay for an advantage early in the game is that they will have that much greater of a challenge towards the end.