I used to be a strong supporter of legalizing doping. I still think it may be the only solution, but it definitely raises problems.
First, not all athletes may want to dope, so most suggestions for legalizing doping concede there will have to be a two-tier system, dopers and clean. But of course there will be dopers in the clean tier, athletes who can’t compete, doped, with those in the doper tier, and seek to get an advantage. So sports at this level will not be very different from sports today, with everyone claiming to be clean, but most probably not. Certainly there will have to be just as extensive testing of this supposedly clean tier as occurs in pro sports today.
Second, sports is about getting an edge, so dopers will always try to find some new substance or program that works better than the widely known and used ones. To maintain their edge, they will want to work in secrecy, which may mean no advice or help from doctors, certainly not from any official medical personnel that a sport may use to monitor the athletes. So much for the “controlled environment” that is supposed to prevent abuse of substances. Do you realize that “control” is basically incompatible with the athlete’s drive to be the best? That by definition you want to separate yourself from the pack, which means you do not want to be limited to the drugs that everyone else is using above board. Hence the situation in the doper tier will also inevitably closely resemble what goes on in sports today, with athletes doping in secrecy and denying to everyone that they are using anything other the regulated substances.
In fact, there isn’t much point in allowing doping if the playing field is level. The whole point of doping is to get an edge. I know, most riders who admit to doping say they did it just to keep their jobs, that everyone else was doing it, so they had to. But if the situation were really that simple, we could eliminate all doping and achieve the same thing. While I suspect there is a somewhat leveling effect in pro sports with regard to doping, the ultimate rational for doing it-what ensure that it will continue to occur--is there is always the possibility of a substance or program that will give you an edge over the rest, at least temporarily. Regulated doping would defeat the purpose of this.
Some may propose totally unlimited doping, anything goes. But not only is this sure to lead to serious abuse, and likely deaths, but by condoning it, sports, it seems to me, will develop an outlaw image. I’m not even sure a situation like this would be compatible in modern societies, where the state is held to have some responsibility for the health and welfare of its citizens. For sure, many people would be outraged by the notion.
You (OP) say: “To me sport is about pushing the limits of the human body. If drugs allow you to push yourself further why not? It's still the human body.” I have some sympathy with that view, I think it is important to see how drugs can change what is physically (and mentally) possible. Yet that attitude is born mostly from competition.
Have you ever heard of doping occurring in non-competitive sports? Do mountain climbers, for example, dope? It seems to me that certain banned substances might help them, and since at least some of these substances aren’t banned for activities where there is no official competition, they could pretty much take anything they want. And maybe some do, for all I know. But I rather doubt doping is very prevalent, because sports like these are mostly about satisfying a personal desire, not proving you are better than someone else. You aren’t measuring your worth directly against someone else. I think an attitude like this is a strong antidote to the competitive ethic. You can constantly push the envelope, challenge yourself, within the limits of an undoped physiology. Indeed in sports like these, there is often great status in denying yourself the aids that are "legal" or accepted, like climbing at high altitudes without tanks of oxygen, or not using standard equipment like ropes and spikes.
Third, there is the problem of youth. Simply passing laws banning substances below a certain age will not solve the problem. Just look at the problem of teenage drinking. Like it or not, sports stars are role models, and kids try to emulate them. If sports are open about doping, of course young kids will want to use the same substances.
Despite all these problems, the current system, it seems pretty obvious to me, is unsustainable. Something has to give.