less,
You are correct, kitniyos themselves are not and can’t ever become chametz; therefore, as just one example, they don’t have to be destroyed or sold before Pesach. Hundred of years ago, the Poskim of were quite aware of this fact but (aside from the Beis Yosef) enacted that neither kitniyos nor their derivatives may be consumed on Pesach. This is because kitniyos are generally grown, harvested, stored or processed in proximity with the five grains and therefore may become contaminated with them. In addition, their flours can resemble that of the five grains and might therefore be confused with them. Once this restriction was adopted, an Ashkenazic Jew may not opt out of it, despite any conjecture that the circumstances surrounding kitniyos have changed, which they haven’t.