Even if Disco itself were preventable, there were enough other sounds that evolved into disco that there would have been something. There was the Philadephia Sound, which was a lot of acts like The O'Jays and The Trammps and producers like Tom Moulton and Gamble & Huff. Secondly, there were people in Europe, one example being Giorgio Moroder, who were playing around with synths. There would have been some form of "dance music" regardless.
I do think there was some racism and homophobia behind the Disco Demolition Night. Steve Dahl, of course, has gone on record denying it. Disco, in my opinion, was a very misunderstood genre that got sucked up by people like the Bee Gees. There was good music coming out that evolved into other styles (Chic, Grace Jones, Moroder were all influential). Sadly, people tend to associate disco with the Bee Gees, polyester shirts, and Travolta pointing at the ceiling. Which is a shame, really.
That's the big key to why people hated disco, IMHO. It's more latent than conscious - very few metalheads and punks even go about espousing rasism. But metal and punk started off and largely remain very white and have remained so.