You are right that nothing is certain and PEDs have permeated endurance sports. However, I do believe that both Ryan Hall and Meb are absolutely clean. Say what you want about their faith, but I do believe they are both genuine in their beliefs and both would consider PEDs morally wrong.
Hall did run a wind aided 2:04:xx in Boston but also ran 2:06:xx in London. While Meb has not broken 2:06, he is truly world class and can put up consistent results which are virtually unmatched (Olympic silver, NYC champ, and a gutsy 4th in London). If either of these guys has EVER doped then I've lost all faith in the sport BUT for now I do think our sport is clean with rare exception.
I agree with you on Hall and Meb.
I do not agree with your final statement. It is probably the other way around. Why do I think so? Simple human nature.
1) There are many, many Kenyans/Ethiopians... for whom prize money and appearance fees are the difference between riches and fame (in their own country) and poverty and anonymity.
2) Even if not all people would take PEDs to go from poverty to riches, without a doubt many if not most would.
3) For most of those who would hesitate the known reality that others are cheating is usually enough to push them over the edge - "Hey, if others are going to cheat then it is not really cheating when I do it. It's just leveling the playing field."
4) The Africans have made a complete mockery of distance world records (and the Jamaicans of sprint world records) over the last 20 years or so.
5) Suddenly the Americans (including Farah) have made this huge leap to beating the Africans at their own game. Is Rupp THAT much more talented than Shorter, Rodgers, Pre, Virgin...? His times suggest that he is in an entirely different galaxy than those folks. It is all too improbable.
6) Drug testing is always behind drug innovations.
7) The kings of the sport profit from new amazing records across the board. What happens to the popularity of the sport (already very low) when records can no longer be broken? Better let them get broken then, by any means necessary.
Sorry, but simple human nature and the evidence at hand make the odds pretty darned high that cheating by drug use is rampant. Sadly, it is hard to see why our sport should be any cleaner than cycling.