Again the evidence is that the very best athletes are able to sustain large fluid losses during exercise (presumably with quite large drops in ECF volume) without any apparent impact on their performances.
More later.
That is a huge presumption that large fluid losses during exercise are mostly accounted for my drops in ECF volume. I suspect that large fluid losses do not see large ECF volume losses because of the buffering effect from fluid shifts from the ICF just like large shifts in acids in the body do not cause large shifts in pH because of the buffering system in place. Unless these different compartment volumes are actually measured during these conditions one is just guessing as to what is going on.
Your arguments seem to imply that it doesn’t matter how much fluid the athlete loses, they will be ok, perhaps even perform superiorly - as long as they “listen to their thirst”. In my opinion, such an approach may work for some but will fail many.