This may be a perspective applicable cycling, but which running pros (athletes and coaches) have ever said "EPO gave me a 10% boost"? I'm convinced there is a reason your first example is Lance. Here at letsrun, we have long had a famous example of a life long professional coach of elite and world record setting athletes who says that athletes can set records clean with altitude training. He is routinely dismissed by fans with little to no credentials.
I tend to take what "enough pros" say with a grain of salt for a number of reasons. The very decision to dope is enough to alter their neutrality, as they become psychologically invested in their decision to break the rules, and look for ways to justify the decision to themselves and look for any returns they can attribute to doping, sometimes lying to themselves, rather than objectively looking at the accumulative effect of other changes and confounders and placebo. But again, no running pros are saying doping helps their distance running. This only comes from reporters and fans and sometimes sour grapes from "clean" athletes.
How can we find out? One way would be better studies, that better control and mimic race preparation and conditions, even if they were on amateurs. But it is not the only way. Another indirect way is to look for trends in performances in the real world. Running makes it easy because the distances are fixed, and to some degree, the best times can be compared. I created two threads which looked for and characterized trends in all time performances among East Africans, North Africans, and non-Africans (+ West/South Africans). While many saw this as confirmation that only Africans are doping to win, the most obvious anomaly was the extreme drought of improvements from non-Africans worldwide in the last three decades universally across distance events from 800m to the marathon, despite the availability and undetectability of endurance drugs and methods that were heavily used in cycling by non-Africans. A country like Russia was not noticeably better than Japan, and a country like Spain was not better than the rest of Europe or America.
Regarding retired athletes telling all, what if those coming out of their career really have nothing to tell and are already being transparent? Who would believe it?