Good point. Does the world champion also have to play through the rounds up to the final match? That could help, having to go through the round of 8/16, so the championship match doesn't end up being the best vs someone who was the best a while ago. Then it'd always be a fun match between the two top. Not to mention drawing to keep your title doesn't seem fun
No; the defending champion automatically plays in the World Chess Championship. The problem with the champion playing in the candidates matches is that the champion and the candidate will play each other before the championship title match, which kind of kills the hype.
Chess is more about the games than the outcomes. While drawing to keep your title doesn't seem fun from a statistical standpoint, the draws, in this case, are often hard-won.
Even more interesting is that one of the reasons that Carlsen has progressed so amazingly is that he has been known to play on in positions where most grandmasters would agree to a draw.
Sports competitions have never been about who the best team is - it's about who wins. This is why it's fun.