no.. he isnt the greatest.... yet. He has to win the WCC first. :)
can we say, right now Magnus is the greatest of all time

He hasn't even beat Anand yet, if Carlsen is able to dominate his contemporaries as did Fischer and Kasparov and Karpov over the long run than yes I would put him up with the greats of all time

Fischer won it but didnt defend it. He did dominate the players of his day like no other so thats why he gets the nod as always being in the conversation.
You are right... Carlsen needs a WCC win and a few defenses to be considered in the conversation.
I agree