The Iifa tree theory makes the most sense to me and I find it very acceptable. It would explain why the Iifa Tree died in the end.

Of course, Necron's words are very similiar to Garland's, except that Necron wants to return everything to zero (which is indeed not what Garland wanted). Could it be, because Garland died, that Necron kind off started to think for himself (if he is indeed the Spiritual Core of the Iifa tree and thus created by Garland(?)) and in the end formed his own theory of his purpose?