So what is your take on diesel fuel prices after 2020?

Being in the petroleum business for 38 years, I've learned a few things. Hydrodesulfurization has been used since the fifties, to remove sulphur, not a new process. Invented in late 1800s, it has been redeveloped/redesigned/upgraded since, to improve octane ratings in naphtha (gasoline), by removing/reducing sulphur in the feed stock.
The price per gallon, liter, bbl or m3 has more to do with supply and demand, less to do with the production/processing.