“Closed primaries, with all states having one on the same day, is the way to go.”
That would solve nothing. Regular people participating in the convention system is the way to go.
Closed primaries would interfere with your agenda of nominating left-wingers, gnat.
Regular people have obligations- and cannot usually just take off on a trip, and in many cases cannot choose when they can do so. In short, regular people generally don’t go to conventions. Closed primary systems screen out Democrats from selecting Republican candidates, and will have the further advantage of suppressing GOP-e left-wingers. Having all the states have their closed primaries on the same day will give those of us in conservative states a real voice in selecting the candidate instead of having to rubber-stamp some northeastern gungrabbing liberal.