There were ~120,000 new cases of Delta on December 3rd. 0% were Omicron. There were ~140,000 new cases of COVID (Delta + Omicron) December 20th. Some unknown percentage were Omicron. If NowCast is right, and 73% of the new cases ending the week of December 18th were Omicron, that means Delta went from infecting 120,000 people per day to something like 38,000 (27% of 140,000) per day. That makes no sense. By contrast, if total case numbers were 440,000 it would make sense, because it would imply that Delta was still chugging alongside Omicron.