Good heavens, the Defenders is a franchise that has struggled even longer than Dr. Strange and Iron Fist have alone. Nobody could keep the magic going after Steve Gerber left it, and it hasn't ever rebounded since ending in the mid 1980's. There are reunions and mini's and whatnot in other books - hilariously, the cast of SECRET AVENGERS are closer to a Defenders roster than an Avengers one - but there hasn't been much in the way of Defenders material in ages.
The difficulty of that franchise is while everyone laughs about it being a "non-team", it actually has something of a consistent roster overall. The problem is that consistent roster are not of big name characters. The Hulk is the biggest name on it, and who knows where the character will end up when his title relaunches. But after that? Dr. Strange, Namor, and Silver Surfer haven't sold an ongoing series in about 10-15 years. And then after them, you have those notable C-List characters like Nighthawk, Hellcat, Hellstorm, and Valkyrie (and to lessor extents, Beast and Moon Knight). Again, SECRET AVENGERS seems to be occupying a few of those other characters.
I don't mind the Defenders, but then again, I never minded MARVEL TEAM UP. That doesn't mean I think it'd sell.