There are a lot of possible reasons George Michael died in middle age, many of which apply to pretty much all men (including you), or pretty much all people living in England. There's also, obviously, a history of drug use; specifically, that he smoked weed like some people smoke cigarettes. Smoking obviously isn't good for your heart either, nor your lungs, which would go a long way towards explaining why he kept getting chest infections.
Under the circumstances, it is extremely unlikely he died of anything AIDS-related. People dying of AIDS don't just go from healthy-looking, functional people to dead in one night, and they rarely ever die of something as mundane as heart failure. He was being poked at in the tabloids for appearing to have put on weight just the other day -- not something you see in people dying of AIDS, but common in people yo-yo'ing in and out of drug rehabilitation, as he is known to have done.
And beyond that, I think George Michael was the kinda guy who probably wouldn't have kept it secret if he did have HIV/AIDS, especially if it were advanced. As you can see in the article you posted, he wasn't exactly shy about discussing the aspects of gay realities that other people don't like to.
Regards the article, I'm of two minds. Some points are salient, yet I detest the simple-mindedness of this particular branch of the sex-positive movement and find it increasingly difficult to associate with it.
But one thing the writer is definitely right about is that our culture is still pretty negative and hateful about gay people trying to tell honest stories about their lives. And that fact is made abundantly clear by your belief that it's impossible for a gay man who doesn't follow the heternormative sexual script to die prematurely of anything other than AIDS, despite all evidence to the contrary.