That world wonder patch will need to be reworked. In addition to decreasing each month based on the gap between target and top 50 alliances, you need to have a minimum amount that the requirement decreases per month. It used to be 13MM points.
what is going to happen is that worlds' point scores could decrease as they drag out due to ghosts/disappearing cities. Have it decrease a minimum of your current change or 1MM points, whichever is greater.
The real solution would be to treat non-allied players as being in a 1 player alliance, so if larger players leave the alliance, their points would still count to the start of WW. Of course, you might still be able to do the same thing with enough small players leaving, but this would be harder to manage.
On most recent worlds, some of the larger players have more points that the 5th or 6th largest alliance, so just looking at alliance points isn't completely relevant.