Here's another idea. What if Jagex, when writing this quest simply said "Oh who am I kidding, we need to get guthix back", and had sliske remove the guthixian spirit part in attempts for some sort of ultimate order of ascension thing.
In the end, guthix comes back as a mortal, and we need to work with guthix to back down Sliske or something, idk.
Maybe he wants our ability to cheat Death so that he can do something that will get him killed, but then he can just respawn? Of course, he references the fact that, after being crushed by the falling rocks, we would wake up in Lumbridge.. which means we still have that power, unless he was trying to tell us that's what he wanted.A
1) If he has a plan, it assumes that whatever we did in his little game made our soul compatible with his (or whomever he had planned to merge the soul fragment with)
2) I agree with Evil Dylan, it gives him 1 more piece of leverage over us. (quest ends, he gives us the location and means of getting back our soul piece back.) Again, assuming he didn't do anything or experiments to it while he had it.
Given Sliske's rather unhealthy obsession with us, he may be doing the next best (but overly complicated) thing he can do to having us: creating a shadow version. He created a shadow version of the WG in DAT. He now has a fragment of the WG's soul. He can essentially create a shadow version of the World Guardian, with most of the World Guardian's abilities and such (at least up to the point of KS, and in whatever way he augments them) and add it to his collection. And this version of the World Guardian would be far more pliable.
I think his words are being taken too seriously, he's saying a little bit of the world Guardian essence is gone, he means our soul, not the power, but I do like this theory, but I do think his word is being taken a bit too serious.