Many fans glossed over his numerous doctors appointment visits leading up to the announcement. Many merely were just overjoyed that they were able to see pics of their man out and about. In hindsight, you do notice the weight loss much more acutely. A casual, 'man he's so thin' has come out of my mouth but because I never would have thought him in danger, I left it alone.
Since his death, I have read numerous interviews and anecdotes from his friends who have said that during touring season, Michael didn't eat. He wanted to, but couldn't. (I'd posted an interview w/ex Sony execs who worked extensively w/him and they gave a vivid visual of Mike when he'd prep for a tour) Basically, he'd put himself into overdrive and placed sleeping and eating on the backburner. If that was the case, there you go.
As far as his "drug use", I don't know what to say, really. Mike under prescription was his MSG concert. His This Is It announcement, I'm just not seeing it. Michael 99.9% of the time wore sunshades, whether he was chilling in Bahrain or going to the bookstore. I honestly don't think there could be any accurate assessment based on that alone. Mike when he was minding his own business, wore shades; if he was trying to 'hide' any drug use, I doubt he'd have his children with him. In almost every pic leading up to that annoucement, he had them at his side holding their hands. Remember that doctor's pic in that rare pic thread where he's not wearing shades? That picture was recent. The only thing I can glean from that is that he looked a bit tired. To me, he was just being the usual cautious father who just happened to have a throng of paps waiting just for him.
I understand what you are saying about only seeing what you want to see. I've been trying, in my own private reflections of Michael, to figure out just how/when/if/why. The main thing is WHY. Why would this happen? The month of May was a very interesting month to me because it seems to be when there was a whole lot of movement in Michael's camp. Firing. Hiring. The re-emergence of shady figures. And then of course all those tour negotiations taking place. I think I can chalk a whole lot of Mike's appearance to STRESS.