The point is that whether we call it 'positive' charge or 'negative' charge makes no difference, as long as we are consistent. If we decided to label the charge of a proton as 'negative' then, to be consistent, we must also relabel the charges of the quarks (i.e. d would become +1/3e, and u would become -2/3e). In which case your question is void.
lemon
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