Just a week ago I had a hen with this exact issue. I had been treating her with every sour crop treatment there is - miconazole, copper sulfate, medistatin. Nothing worked. I even resorted to extracting the liquid from her crop each morning. But it became apparent that an underlying issue was causing the crop issue, and since it was not getting better, so the crop couldn't recover either. The hen died the night before I planned to euthanize her.
So, yes, when nothing is working and the hen is getting worse, not better, then euthanasia is what I suggest.