Olympus periodically kills 5% of defending units within its walls. This does not include neutral units.
"within its walls": As far as I can remember, the temples and Olympus are supposed to have no walls, therefore this sentence might be misleading. It also implies that only land units are affected by the curse, while in a former DevBlog article the curse was described as affecting all stationed units.
At the moment I cannot see how any alliance could (re)produce enough defensive units to hold five small, two large temples and Olympus, given the fact that there are no walls (and additional losses due to the curse in Olympus), and at the same time (re)produce enough offensive units to conquer Olympus at least three times (or even conquer temples/Olympus from another alliance). Additionally, almost all (small) temple buffs relating to units are attack bonusses.
Even if you think it possible that all alliances holding Olympus have portal temples of their own and will send their main defense only when lots of attack movements are coming in, their defense units (once withdrawn again) will have to take the long way home and will not be available for the next wave of attacks coming from an alliance holding a portal temple as well.
I must admit that I really dislike the idea to introduce a new endgame featuring so many NPC elements, especially as it will bring all normal game play (PvP) to a halt. Of course it is interesting to see a new concept developed and being tested, but I have been with Grepolis for so many years because it is a PvP game. I even hated the Artemisia event, although then you had only to use offensive units to conquer those NPC cities, while now also defensive units will have to be involved, and the alliance using the most gold for instant troop rebuilding will win.