Jackson helped ruin modern music by steering it away from being able to play an instrument and sing in the direction of dancing and choreography, which simply looks foolish and embarrassing, even when done by people who are "good" at it. Moonwalking and doing that impossible looking leaning thing that Jacko did are more analagous to tightrope walking or juggling than music. Jacko was more suited to a circus than a pop show. Because of him you now have groups and solo artists who move about to disguise the fact they lack the talent to play their own sounds. We've always had a few like that (Jagger etc) but they used to be the exception, not the rule.
Assuming the candidates for this poll have to be dead I am puzzled as to why Jim Morrison is on there. Buddy Holly, George Harrison or Jimi Hendrix are more deserving, surely?
The argument if there'd been no Lennon there'd be no Jackson is not really fair though. If there'd been no Presley there'd be no Lennon. Everyone draws some influence from what went before but I do think John took music far further and explored many more new directions in the time available to him than any of the others on that list did in their own times.
But then again I'm biased