I don't believe that the anime has been on-air for too long. That's not why it should stop. Look at shows like Doctor Who and Familiy Guy. Both of those have been running for years, and they both are still successful, and, based on the general consensus, entertaining.
Time is indirectly the issue here. Because the show has been on air for many years, ideas have been stretched thin or just lost. And no, the reason isn't that I'm not the target audience. I can watch the older episodes and still be entertained. Same issue here with Spongebob or the Simpsons. I'm a huge fan of both, but they totally fail to deliver. What I'm trying to say is, the show shouldn't be taken off the air because it's been running for too long. But because it is running too long, it has gotten progressively worse, and that drop in quality is the issue.
I have claimed Alduin, the mighty dragon! He shall one day return to bring forth the end of time, and fulfill the prophecy of the world eater.