Because that ability does not need to be refuted. A stunned DH is every bit as “made-of-paper” as any enhancement shaman. You want to easily dumpster a Havoc demon hunter? Coordinate stuns. Simple as that. Even a modicum of cooperation will result in a very dead demon hunter. Mages, meanwhile, can just decide they don’t feel like being stunned and blink right on out without even having to touch their trinket. Warlocks, paladins, warriors, DKs, druids, and most other classes are durable enough to survive being stun cycled as long as the rest of their team doesn’t abandon them. The DH, meanwhile, is most likely going to die even if a healer is pumping everything they’ve got into saving them. A stunned DH also won’t be using blade dance.
As for blade dance itself, you are greatly overestimating its worth. You get a 100% increased chance to dodge for one second. Sounds strong, right? Not when you consider the number of abilities in the game that just can’t be dodged. About the only thing it really completely screws over is warrior, but even they still have a few things that get through it.
Are you familiar with logs? My opinions are based solely on math. Havoc’s ability to perform well in solo content is a non-issue. Havoc isn’t top damage in any area; upper middle of the pack single-target, top five or six (but not top three) for AoE. They also aren’t overly represented high up on the PvP ladders, and random unranked BGs are totally irrelevant to start with.
Furthermore, your belief that mechanically simple classes should perform poorly would essentially entail condemning nearly every class and spec in the game to poor performance if ever put into practice. Have you considered this? You choose to target DH, but there are equally simple specs that output much higher numbers.
Another thing you are failing to take into consideration is APM. The number of buttons you have available to you does not necessarily dictate APM. Warlocks and mages, two of the classes with the lowest APM, are also two of the highest performing on single target. The latter is also generally in the top two for AoE. Frost DK is another example of a very simple class (even more so than DH I feel, though my opinion may be skewed by years of playing a DK) with relatively high APM. Should it have its performance curtailed because of its simplicity?