What are you talking about when you speak of good/best outcomes? I think especially when it comes to talking about fitness it is important to be explicit about outcomes because only with outcomes in mind can you talk about what “good” actually means.
In that context I’m a bit confused by your focus on (endurance) athletes. Their overriding goal is obviously increasing their athletic performance in a way that keeps them injury free. Their main focus aren’t health outcomes. That’s not the same. So if you focus on them you are in danger of talking past each other.
To that point: sure, those studies often have unrealistic protocols but the way (performance) athletes train is also unrealistic for most people!
If those studies are about population health they should have realistic protocols (in terms of: realistically achievable and adhered to by non-athletes, in the best case also empirically grounded in the sense of: this is a realistic range of activities non-athletes might actually engage in on a regular basis) for non-athletes.