An EMVR produces a MERC only after two emission tests. The second test determines the car's MERC value. In Texas, the MERC value also depends on a repair estimate. In Texas and California, cars must be insured, operable, and registered in the target area to be eligible. Illinois only seems to require a title check.
Cars used in the winter probably have a lot of remaining life.
A mechanical screening is part of Illinois' recommended rules, but a car's condition does not influence the expiration date of the MERCs it represents. Unless the newer cars' emission control systems decay more rapidly than other parts, there is no reason to ever update the model year constraint. EMVRs and attrition will gradually make EMVRs obsolete. The difference between the emissions of post-1981 clunkers not worth repairing and average replacement cars won't be large enough to generate significant MERCs. In most cases, vehicle characteristics did not affect price because of concern about tampering.