I don't really think Protoss is underpowered at higher levels. It's just this:
A lot of the community defined pros are actually really good, and take their time in analysing games and replays.
A lot of the self-proclaimed pros that choose protoss have cheesed their way to Master/GM, not knowing much about protoss, and when their cheese fails on repetitive players, they start to lose out.
That's the way I look at it. The race is not underpowered by any means, MC, Naniwa, incontrol, KiwiKaki, Tyler and axslav off the top of my head really have shown great results in tournaments because they understand the race. Saying that protoss is underpowered because master level+ protoss find it difficult to cheese masters+ doesn't mean the race is imbalanced.
I'm not saying that for sure they're balanced or even over powered, but I think a lot of the protoss players don't actually understand their race to actually be effective with it, leaving only a select few pros to actually think of new builds to win games convincingly.
Right now I think Zerg and Terran have a lot of true professionals looking at the race while Protoss have only a few, because of the lack of player skill.
But I say this, Just because Protoss aren't in the top 10 of any GM league, doesn't mean they're UP. I'm sure a lot of Protoss actually practice on customs rather then on Ladder.
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