I hate this argument, because it's worse than vacuous: It makes out that art is about as meaningful as a gunshot in a crowded room. Is that art? It'll damn well get a reaction, especially from the person who was just shot. Shouldn't we demand more from art? Like actual meaning and maybe, just maybe, a positive impact on the world?
Or should the next art piece be a car bomb in front of the Louvre?
Yes, burning a million pounds is comparable to a gunshot because a million pounds could save a life, so destroying a million pounds might well have ended one, in terms of opportunity cost. And no, you don't get to handwave that away by pointing to other people doing things, any more than a serial killer gets to exculpate themself by pointing to tobacconists. People do things with consequences, and then have to live with those consequences, and the members of the KLF are getting off easy.