1. It's quite normal that performances improve with huge margins when technology, training, equipment, dieting, professionalism, available money, being able to train full time etc all gets way, way better.
2. Not every kind of doping that is being used can be caught by labs. An athlete could be using a substance in such a way that it's nearly impossible for labs to find it (yet). Which is why retesting is important. We see that now, with a whole bunch of cheaters of 2008 being caught.
3. You mean non-Western labs are cheaters too right? Because they suddenly make athletes negative, like the Algerian decathlete.
4. I agree (unfortunately) that I think most medalists are not clean. Which is sad, but probably won't ever change.