I just looked at the list of long runners on Playbill. The top four longest running shows (in order) are Phantom, Cats, Les Miserables, and A Chorus Line. Now, the only two shows that I would consider to be pretty god are A Chorus Line and Les Miserables. I can't stand and never got what it was that drew people to Cats. And, I will admit, that Phantom, although a guilty pleasure of mine, is not the best thing ever written nor the best show out there.
However, I think that a show has to have some people who like it in order for it to sell tickets. I mean, if there wasn't a bunch of people who thought that Cats was amazing and that it was the greatest thing since sliced bread, it wouldn't have had the longevity that it had.
"If you try to shag my husband while I am still alive, I will shove the art of motorcycle maintenance up your rancid little Cu**. That's a good dear"
Tom Stoppard's Rock N Roll