Even if you ignore fraud, national mail in voting has a nightmare potential to introduce layers of complexity to the basic process of voting, and multiplies error points along the way, from the distribution of ballots to the delivery of ballots to election officials, to the counting, recounting and certifying of ballots.
Besides, it’s lunacy to consider changing the vote system scant months from a highly charged election. All we’d need is a fuse and...kaboom! Open warfare!