I don't think that he would wait until the convention. That wouldn't give another candidate a chance to introduce themself to the country during the primaries. That's what primaries are for: to introduce candidates to the voters. I hope that Biden will come up with some lame health excuse, drop out of the race before the first primary, and endorse Harris. That would allow other more viable candidates to run because, of course, Harris wouldn't have a snowball's chance in hell of winning the nomination. She is more unpopular than Biden.