SE-0309: Unlock existential types for all protocols

Excuse me if this sounds silly, however, could I ask why any P is considered to be needed (in the future or as an alternative)? Read proposal and Improving the UI of generics but still confusing to me.

To make type declaration clear that "A protocol cannot be conform to itself"?
In that case, why some P is not sufficient?

(I'm not against the introducing the new keyword at all, but would like to understand them)

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