MK is boring.
Brawl, to me, is cool because of the complicated network of ifs, ands, and buts that happens when one character (or both!) is off balance. The most common complaint leveled at Brawl from people who base their arguments on game theory is that the defensive options are so good that the game always reverts to its base state with just two characters separated and trying to approach each other. This complaint ONLY applies to games of Brawl which feature MK.
Approach MK from above, below, or the side, and MK always has an instantaneous, long range, disjointed attack with medium-high knockback and negligible lag. There is no direction which is better to try to combo him than another. If something isn't a 100% guaranteed combo, MK always has a combo breaker which doesn't put him at any risk afterward. His light weight helps remove many of the true combos, too. Approach him when he's off the stage, and he still has all the same options he did before. Approach him when he's been in the air for 4 seconds and it's no different than if he were in the air for 1 second and only barely different than if he's on the ground.
Playing MK is definitely a test of skill. It is, however, always a test of the same skill, as MK's options are never any different, and he is never off balance or in his strongest position. There's no reason to try to box him into a particular position because there is no better or worse position for him to be in than the one he's currently at. Strategy never enters the picture, unless you count MK choosing a time to start stalling, which isn't supposed to be a part of the game anyway. Without any strategic interest to it, the game is exceptionally boring to watch and quite difficult to justify playing.
Brawl will have a longer lifespan without MK than it would with him, partly because Brawl is objectively speaking, from a game theory point of view, a better game without him. Frankly, even if MK sucked, Brawl would still be a better game if he were removed. That MK is the best character simply makes the problem a defining feature of the game, to the point where the game's flaws which only pertain to him are frequently generalized as being characteristic of the game itself.