If FedEx and Amazon set up private airports dedicated to these machines, only put cargo in them, and make their flight routes distinct from other aircraft, I imagine this could be up and running faster than people think. Edge cases shouldn’t result in loss of life, so getting through regulations and insuring should be much easier than other situations.
For example, there's Wilmington Air Park in Wilmington, OH: https://www.wilmingtonairpark.com/
Apparently, Amazon started flying through there in September of 2019: https://airportimprovement.com/article/wilmington-air-park-w...