The manufacturers are global entities. Before final tier 4 came out, the manufacturers got wind that Europe was going to tier 5 emmision standards.
Rather then reinvent the wheel. Manufacturers made sure the Final tier 4 here in the US would meet Europe's tier 5 requirements.
So even if we followed Europe's lead with tier 5, all the equipment manufacturers would need to do is change the badging to tier 5



Even if the US lowers restrictions, the equipment manufacturers will leave the equipment as is.