He's very squishy early game. Until he stacks some of those souls your AA's are going to hurt. This will always take time.
Since he requires champion kills to function. He'll need a kit that can ensure kills. Not just catch people and burn flashes. He needs to really hold a champion in place with harder CC.
But on the other side in modes such as Dominion or ARAM where kills are always plentiful he'll be OP with this change.
Since high end game have low kill numbers, he'll need stronger base armor and maybe armor per level to compensate.
Currently his souls don't have reduced effect for multiples. Each one is worth the same value.
70 armor in 20 minutes(actually a very forgiving number, he probably wont be getting this number unless he gets it before he start roaming) is not enough to cover what he gives up.
Comparing him to the other two tanky supports, Leona and Braum, he has 10 less armor at level 1. He has no defensive steroids outside of a weak shield that he probably isn't even going to use on himself, and he has no scaling armor. Sure, he is ranged. But compare him to Annie now. She also has an armor steroid like Leona and Braum. She also has more armor than both at max level. Her starting armor is lower than them, and 3 higher than Thresh.
Basically. His souls are his scaling armor. He's not even likely to get enough to make up the difference. But he gets AP(damage) with them and if he does get enough he can go past what he would get if he had scaling armor.
It already has a lot of risk to the reward, because he could easily fall behind for the lack of scaling armor in a teamfight. If he's having a hard time in lane it's even worse as that's where he gets most of his stacks.
