Of all the terrible things that the Trump Admin has done, this is perhaps the worst of all, with the gravest repercussions for humanity.
But who cares about science, or humanity for that matter, so long as big companies can increase their profits and keep greasing the wheels of corrupt politicians!
I doubt it will end up accomplishing much more than letting 'delete' kits go legal again which have relatively weak penetration into the market. 3 years isn't enough runway to start manufacturing things without emissions unless they can get green-light to import foreign models that fast, which due to protectionism will probably get delayed as much as the admin can.