Peter...obviously if no one who plays it thinks it is great, then it is not great.
But from what I know and what I've read and studied, a truly great course can and will entertain a true golf enthusiast for years and years and years...and not just the scratch golfer, the average golfer as well. Think Mackenzie and The Old Course. MacKenzie from my understanding wasn't a good golfer (at least for the majority of his life), but he was arguably the greatest architect ever to live and certainly knew greatness on a golf course. In The Spirit of St. Andrews, he says that even after playing TOC for 25+ years, it was still showing him new nuances and ways to play it...this in his mind made and/or kept it great.
I've heard Tom Doak say comments about courses (Seminole I think is one) that go something like this...if someone doesn't think this course is good, they don't know what good is. Like MacKenzie, I think Tom D. knows what makes a good/great golf course.
So, if NO ONE thinks a course is great, then it probably isn't. But if some people think it isn't great, that doesn't mean it isn't. It all depends on who those people are and how good they are are recognizing greatness.