While I agree with the main points (which I will explain later), it is not true that you only draw blood if you want to manipulate it.
Every elite team and college team draws blood every few months, to check for:
1) thyroid conditions - these need to be treated asap, or the athlete will fall apart in academic life AND athletic performance
2) iron status - especially with females needs to be addressed immediately
3) vit D status - important for all
4) anemia
5) CPK values, to make sure the muscles are recovering appropriately
6) general health values, like blood pressure
So you see even without intention to doping/use illegal stuff, blood needs to be drawn. The thing with Salazar (and Dr Brown) is, he did not just treat deficiency's like any other team, he tried to optimize all values regardless of there being a deficiency or not, even if it comes at a health risk for the athlete (for example, thyroid meds without conditions can lead to hyperthyroidism and Ritz and some female athletes actually got these conditions and felt terrible under Salazar/Brown's treatment).
So far we are still legal. However, he also optimized testosterone levels (always testing for it, and increasing it illegally) and did massive L-carnitine infusions. People wrongly assume that he only gave Magness a 1-liter infusion, and stuck with 50 ml infusions to everyone else (which would be allowed). Truth is, they also did 1-liter infusions with elite NOP athletes, but changed the records (there is proof for that) stating it was only 50 ml. It doesn't take a genius to figure out that changing records, and doing the infusion with just Dr Brown+elite athlete+Salazar makes it 100% untraceable. Furthermore, we only know what Salazar did what athletes talked. Rupp and Goucher were definitely not the closest links to Salazar, so he never exposed them to the really hard AND secret stuff he might have used on athletes like Rupp, Farah and Hassan. These athletes obviously won't talk if they did anything against the rules, and no one has a chance of finding proof. The WADA and everyone who follows the sport knows what's going on, we just can't provide concrete proof because they operate in secrecy and keep their mouths shut (ever heard about a convicted doper? They also NEVER admit they took something, even after multiple positive tests, see Kiprop et. al.).