I am not saying that allowing everything is the best solution, but I do see some similarities with the recreational drug prohibition.
there's a big difference with recreational drugs : nobody will get any recognition or reward for taking a recreational drug, it's not an activity which is valued by the society because it's associated with pleasure, not with performance. It's considered as a little bit shameful activity, even when the consumption is allowed (and I'm rather in favour of this allowance for drugs like marijuana). And there's an addiction problem, but at least people are aware that it's bad for themselves and their body to abuse those things.
My problem with allowing PEDs is clear and it's not only their danger for the health as you seem to think : as soon as you've decided that, and even if commercials are more or less forbidden, you will read everywhere plenty of articles praising the advantage of this or that PED for the common person, everybody will want to increase one's performances through this or that. Humans will be living pharmacies. I know that there's already an evolution this way, especially in the USA, and I'm not surprised that you seem to come from the USA saying that. But I don't think it's a good direction for the world and human beings in general.
High-level sportsmen are models for people : if they use this or that, other people will want to use this or that.
As for the rest of your message, the notion that not everything can be forbidden ... is very well-known by the anti-doping authorities, they have debates and rules about it, there's no ignorance problem about that from the anti-doping authorities.
How much harder would it be to administer a controlled programme which individually monitored each athlete's health and performance without preference to any individual?
Biologists are able to see anormal deviations from an athlete's standards, that's the main interest of the biological passport.
These guys need to practice 4-6 hours a day, plus off court work. They need to be prepared for matches that can last for 4 plus, recover for the next day and are screwed if the get seriously injured and this is on a weekly basis for a season of 10 months. And Tennis is not even among the most physical sport out there. The human body is not design to substain this, the human body can easily breakdown and this the tool for their professional living that they depend on.
the question of the intensity of what's required from them is clearly raised, in tennis but also for instance in football/soccer. That's clearly a big challenge for the notion that they mustn't dope and clearly a reason why once doping is developing, most athletes "have to" use the same PED to be able to follow the same regime.
That's why many people concerned about anti-doping ask for sports which require less intensity from their sportsmen. In tennis, for instance ask for quicker surfaces and a smaller calendar. It's clearly a topic which anti-doping authorities are very much concerned about and have some things to say.
So I also think is close minded and prejudicial to think that these athletes should only use bread and water and ignore science developments that so clearly can help them train harder, recover better from injuries etc.
it's not close-minded, the choice for so much intensity is not a given, it's a choice which anti-doping authorities and also some athletes do question.
I know the argument that if you allow this people will search for more dangerous substances I am not so sure though.
this argument is much more obvious for PEDs than for recreational drugs. For recreational drugs, people may want "more" but at least there's no immediate question of performance and competition to be "better" than the others.
In sports, competition, the argument of increasing performance, looking for more and more, is the key
of the activity.