I like the system as it is. It lets you decide in advance whether to fight the AC, or give in, sacrifice a city,
No, it doesn't "let" you. It
forces you
As has been pointed out, you have to have already adopted Ashen Veil and researched Infernal Pact. That's two techs off the normal path, and THEN the huge

cost, and THEN sacrificing a city.
50 turns for anything is obscene in a normal speed game. I've seen the AC rise from nothing, to demons swarming the world and wrecking everything, in far less time.
And just because the demons stop attacking you, doesn't just make the armageddon problem go away. There's still the colossal unhealthiness from blight to deal with, and when you're AV, hell terrain spreadsto you first, so wave goodbye to those health resources., As well as global damaging at periodic intervals, the Avatar of Wrath stealing half your troops, the apocalypse killing another half, and let's not forget the horsemen's global effects, massive unhappiness, unhealthiness, relations penalties, and lowered city defence,
My main problem is that with a 2000

cost, you're forced to make a choice a LOOOOONG time in advance, before you have any evidence of which way it's going to go. And for those 2000

you could do a lot better by converting to order and spamming 2000

worth of crusaders to fight off the demons.