How exactly am I stagnating by not lending plausibility (and thus some tacit acceptance) to a demonstrably self-limiting belief absent absolutely iron-clad evid

How exactly am I stagnating by not lending plausibility (and thus some tacit acceptance) to a demonstrably self-limiting belief absent absolutely iron-clad evidence in its favor? I'm not a research psychologist who makes a living or derives self-worth studying expertise. Remember, I'm not completely discounting talent and fostering unrealistic expectations. In fact, I explicitly stated I believe talent is necessary to succeed at the highest level (i.e. medaling at the olympics). I just think even if you are un-talented, if you do everything right you can still be in the ~97-99th percentile in terms of ability (depending on the activity) which is objectively still pretty awesome.