I understand what you mean, compared to other Win Conditions it seems kinda out of place. In our games usually though, you can only get a Diplo Victory if you full kill another civilization, so that you have 1 more player than your opponents to vote during World Congresses. Apart from that Diplo Victory is only possible if your opponents completely disregard the Diplo Point Options in Congress for the entirety of the game/ and not bother denying the Wonders (Mahabdo Temple Statue Liberty etc)