Right, so Zoro's role is also combatant. Good that we both agree here.
Why should the guy, whose ONLY role in the crew is to be the combatant, be LOW DIFF material to Luffy? It is acceptable for other guys cause their main role isn't combatant, but humiliating for Zoro cause fighting is the ONLY THING he does.
Even Oda realized this back in the days, which was why zoro used to be relatively close to Luffy pre TS.
Being weaker than the captain is understandable and expected, it is the gap... Look at the other partner type RHM Roger/Rayleigh, WB/Oden(prime) and then Luffy/Zoro, and tell me you don't see the problem here.
