Positive intent is a somewhat subjective term. I prefer the term good faith.
Assuming good faith does have two downsides. Firstly, handling in good faith isn't the same as handling in your best interests. Secondly not all people are handling in good faith, and some collectives are subject to selection bias.
Assuming good faith does have two downsides. Firstly, handling in good faith isn't the same as handling in your best interests. Secondly not all people are handling in good faith, and some collectives are subject to selection bias.