The problem there is with DiploVictory in general, rather than with the AP obsoleting.
The "Diplo Victory Vote" needs to be a "World Government" Vote
Voting Options:
Current Leader
Candidate 2
Abstain
Never!
If one candidate wins, there is a massive change in the UN/AP (whichever it passes in)
1. If you Defied the Vote you are Permanently outside of the UN/AP
2. UN/AP Resolutions are no longer voted on, when the Leader chooses a Resolution, They and they alone chose whther it passes or is revoked
3. If you did not Defy the "World Government" Resolution, you cannot ever Defy any Resolutions again.
If all the Defiers of the "World Government" are eliminated or vassalized, then
Everyone who voted for the Leader gets a Diplomatic Victory.
Defying the DiploVictory should be fairly common for the AP, because the forces arrayed against you would be 2/3+ of the religion.. not 2/3+ of the world.
Also there would be a Reason to vote for someone else in the UN, you would effectively voting yourself a win (if That civ won... if you vote for the loser, you lose)