To be honest, if Conquerer's Haki only ever belonged to people who had no one above them in the hierarchy I would be fine. That would have been logical.
But that is not the case. BM and WB has sailed under Xebec, Rayleigh, Shanks and Oden have sailed under Roger, Ace has sailed under WB, Katakuri sails under BM and so on.
Hence I don't see the point of this debate. If the discussion is that it is fundamentally not right for Zoro to have CoC because he is not the Captain, then it's okay, we can discuss how giving Rayleigh, Oden, BM, WB, Shanks etc. was fundamentally incorrect choice by Oda. But if the discussion is that Zoro won't have CoC now because he is not the Captain, then it is not okay, because we have already seen numerous people who weren't Captain have it.
Zoro having CoC doesn't undermine Luffy. In fact Zoro or any other SHP not having CoC would undermine Luffy, because it would mean that Luffy was never able to bring a Conquerer in his crew whereas Xebec, Roger, WB and BM all were able to do it.
If a Luffy fan believe that Zoro (or any other SHP) shouldn't have CoC because Zoro (or other SHP) isn't captain, even though there are numerous examples in OP world, he is a sorry excuse for Luffy fan.