I just don't agree that "40+ player throwing more fastballs than curveballs = more likely to be juiced". A guy throwing like Roger Clemens may very well be using more energy, but a guy who throws like, say, Greg Maddux is still putting a huge amound of stress on his body- and certainly more on his joints. Don't forget that Clemens, in his later years, had lost a lot of his velocity and his ability to go deep in games, and relied a lot more on control than he did on velocity. If the guy was 45 and still throwing 95+ for seven innings, I'd be with you here, but that wasn't the case with Clemens.
Even though he did, in the end, turn out to be juiced, I simply do not see how throwing fewer breaking balls at an advanced age makes a guy more likely to be juiced.