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> Shouldn't it be easy for some large firm to replicate your approach and make the alpha disappear

First, I don't think alpha ever fully disappears.

Second, after speaking with twenty or so firms very few are using sentiment directly. Those that do, I suspect don't take the additional steps to build complex NLP based systems and weight insiders/experts. Even if you weight experts, the methods for doing so are also complicated (cross check against LinkedIn should be easy enough, but also limits information).

Anyway, I personally haven't seen much difference in the back testing.