I don't see why mass would phase themselves out of the HUGE govenrment $$ paybacks they get for highway use by opting themselves out of the emissions program.
Something similar is coming up in the 'attainment area' in southeast Wisconsin, post '96 OBD II engine systems can be plugged into a glorified laptop and given a pass/fail, pre '96 stuff requires dynamometer time and exhaust sniffers and trained people to operate the stuff
It seems to come down to money, and the pre-'96 vehicle population up here in Rust Heaven is falling fast enough they think they can still make emissions-numbers good enough to keep the EPA off their back