Genocide, much like Collusion is not a crime.
However the premeditated mass killing of civilians certainly is a war crime. That is what Trump threatened the Afghans with. Proportionality is a basic part of the Law of War and while Trump may think he can do whatever he wants, he would be committing a war crime. I attached a link to the DOD Law of War Manual so you can, if you like, determine which one(s) he would be charged with or how he would be exonerated.
The only plan like this that exists is in Trumps head.
As far as the feasibility of what he claimed to be able to do, it is not possible (we do not have enough bombs to totally obliterate Afghanistan), strategically insane (using all our bombs would leave us vulnerable), if you did bomb the urban centers you would leave the 22 million people in rural areas without a central government (Do you think that would radicalize them?) and therefor stupid to a Trumpian degree.
I think we should have less involvement in the Middle East. The importance of the region is much less than it was 20 years ago.
However the war in that area is not over and withdrawal is more complicated than maintaining the status quo or winning.
I have seen no evidence that Trump is capable of the required attention to details, the intellectual investment on the part of the commander in chief, and that he has a clear-eyed understanding of both our capabilities and the limits of our power to accomplish it.
I don't have to paint The Citrus Caligula as a horror show, he puts it on every day in this, the first ever, Presidential reality horror show starring Donald Trump. Coming to you on TV and Twitter.
https://archive.defense.gov/pubs/law...-june-2015.pdf