If I indeed think that oath is, as you said, an outright stupid anachronism, then why should I consider it poor conduct to openly reject the oath?

On the other hand, I think it's the appropriate conduct for anyone who wants to be a political representative of me, because I am an anti-monarchist. I do want my representatives to falsely affirm the oath, only because if they reject it then they can't represent us in the electoral system. I see no positive meaning in that oath, no honour in upholding it, no hypocrisy in betraying it.