After 1969, freshman COULD wrestle, whereas before, there were rules against it. Pretty clear line of demarcation in regards to historical significance. No other changes, as of this moment, impacted eligibility and the sport's records like that change did. The exceptions are the 1947 year of freshman eligibility and the additional COVID year.
The fact coaches became the arbiter of who wrestled when is immaterial - they were not allowed prior, making it a logical place to separate eras - the three-year era and the four-year era. Not sure why it's so bothersome.
One other separator could simply be 1973-74 - the Divisional era, where the NCAA went to three divisions.