15% is a pretty low bar in a winner take all election, and being able to possibly win seems like a reasonable standard if the goal is to have a real debate. In that case I would include the top 5 polling candidates, no matter how low their percentage is. That way there is a chance for sokething to be important other than the talking points of the two major parties.
I don’t think it would be better, but then again the ‘debates’ are pointless any as they are just another place to spout campain slogans, and no actual debate occurs.