I think the 2-year belt rule is a case of aftermarket support from NHRA.
Same deal on trans blankets, isn't it ?
I've, um, tested a 20-year-old OEM shoulder/lap belt, though at about 30 MPH, it did its job.
And its retractor also worked as intended.
I'm not arguing that the OEM belts are as good as race belts, just saying that the "exposure" argument seems contrived.