Mo Greene ran at the top of the world for about 10 years. Heike for even longer. Same with Marion. In all cases, there PRs were achieved in the middle of this run (which is to be expected for anyone doping or not). All were dopers. Mo only dropped off after a series of injuries / surgeries that were largely undisclosed.
As for the masters analogy…that’s an unfair comparison. Those guys aren’t training and competing at the highest levels like the pros are during their 20s and early 30s. That training takes a toll on the body that can only be withstood for so long before the injuries start to mount and big dropoffs occur (whether doped or not).