Even though Darth Vader was meant to be his first and last name when George wrote it, the phrase, “You can’t win, Darth” still makes sense if Darth is a title instead of a first name. Can people not be referred to by just their title alone?
“I’m not feeling well, Doctor.”
“You have failed me for the last time, Admiral.”
“You can’t win, Darth.”
And like you said, Obi-Wan could be mocking Vader by reminding him of what he has become. In my opinion, instead of fixing this line, characters in future stories should just sometimes refer to other Sith Lords as ‘Darth’ to establish that it is a valid way to address a Sith.
But I know this is a thread meant to discuss changes that could be made for the OT, and I think this could be addressed just for the fact it is a common critique. It is more of a retcon than a continuity error, because I think that phrase is grammatically correct either way.