There are already plenty of efficient cars on the lower-end used market. The threshold for this program is 18 mpg, which means most small and mid-size cars don’t qualify. Even my old 1998 Dodge Caravan was too fuel-efficient to qualify.
As a whole I think it’ll work correctly, but I’ve learned if there’s a buck to be made, someone will always figure away to do it, either honestly or dishonestly