Since the observation I was making completely addresses Minnesota's absence, your rephrasing is an incomplete summary. It isn't that the tiers became locked beginning some unknown "decades ago" but that they specifically became locked sometime after 1970. I've always acknowledged that it's *possible* that this "locked tiers" claim won't be demonstrated forever. My main emphasis is that, since 1970, this has consistently been our only finding. The helmets in the 70s and 80s have perfectly carried over to become the helmets today -- not one has fallen out.
I didn't mean to misrepresent what you said. I couldn't remember your exact timeframe so I just said "decades" because I thought that the accuracy made up for the lack of specificity.
Even there, I still do not agree. First, I noticed that you phrased it specifically as being inclusionary rather than exclusionary. Ie, you stated that no helmets have fallen out since 1970. I would note, however, that Florida, Miami, and Florida State have all, at least arguably, joined the group since that time.
As far as falling out, like I said upthread and in our previous discussions of this, I do think that it takes a REALLY long time. I also believe that helmet status is much easier to maintain than it is to attain. Thus, Michigan doesn't need to win NC's to maintain helmet status but Wisconsin does need to win NC's to attain helmet status. That said, I still maintain that nothing is permanent and that current "helmets" can and will fall out of that group.
Since winning back-to-back Conference and National titles with back-to-back undefeated seasons in 1940 and 1941 Minnesota has won only one NC (1960 and it is a very weak claim) and only two conference titles (1960 and 1967, both shared). That is one NC and two conference titles in 76 years (1942-2017).
Nebraska's last NC was in 1997 and their last conference title was in 1999 (B12). If the Cornhuskers only win one NC and two conference titles in the 76 years from 1997-2072 they will clearly no longer be a helmet as of 2072.
Michigan's last NC was in 1997 and since then (inclusive) they have won five conference titles (97, 98, 00, 03, 04). If the Wolverines only win one NC and five conference titles in the 76 years from 1997-2072 they will clearly no longer be a helmet as of 2072.
Ohio State's last NC was in 2014 and since then (inclusive) they have won two conference titles (14, 17). If the Buckeyes only win one NC and two conference titles in the 76 years from 2014-2089 they will clearly no longer be a helmet as of 2089.