Earlier you said "we know something produces performance gains". This is where the problem starts, because this grossly overstates what "we know", when it comes to performance gains in distance running events, especially for the men.
I'm all for long term controlled studies when the right studies are done properly on representative subjects. There were recently (at least) two meta-studies that reviewed decades of blood doping performance research which concluded that most performance studies were sub-standard (e.g no blinding, or no controls), and from the few that were not, they did not support exagerrated performance benefits you suggest "we know". I'm also not convinced WADA is motivated to determine whether performance enhancement from doping is real. In fact, finding out some doping doesn't always "work" will only muddy their message and sow doubt in their supporters. It is better for WADA to keep some mystery and mythology alive. WADA's criteria for banning a substance is the low bar of "potential to enhance performance" -- such an assessment made subjectively each year by an "expert panel".
While real world trends might be confounded by supershoes, track improvements, training improvements, diet improvements, opportunity improvements, etc., the lack of any performance improvement trend where predicted speaks much more loudly. The most notable thing I found in my two all time performance reviews was the near universal lack of any trend of significant improvement among the largest set of non-African populations, many of who were known to dope. Depending on the event, non-African men generally peaked in the 1980s, or, in the marathon, by 2000. If doping worked as advertised, we should have already seen significant performance improvements worldwide in the 1990s, rather than just a few regions in East and North Africa, and a few women in China and Eastern Europe/Russia. Furthermore, in an (unofficial) assessment of 12 years of IAAF blood doping test results for 5000 athletes, Kenyan and Ethiopian blood doping was below the global average, and they were not even in the top-10 worst blood doping suspected nations. Russia was the top by far, and they have no Russian men running faster than the stars of the '80s, and fewer Russian women than Japan, one of the least suspicious doping nations there is.