IMO, I think you are, in the main, correct with your observations above. I do disagree though that "the percentage of 4 and 5 year olds is higher now than in the past...". For our area, and multiple other areas that I know of, there are now significantly fewer older bucks alive than even just a couple/few years ago, FWIW. But...keep in mind that the genesis of this thread was "Giant deer of Iowa are rapidly becoming a past memory". So, the thoughts expressed here are pretty on point to that topic, IMO and I think, reflective of so many guys experiencing this drop off AND being quite concerned about it.
Speaking as someone who has commented about "...4 year olds getting shot that should have been passed. ..." I do realize that the notion of an area being under such "management" isn't the reality everywhere in the state, for sure. But...it was pretty much a reality for the past 20 years or so in some areas...so those of us that experienced that "boom" are now lamenting the disappearance of the same. Again, FWIW.
Your last comment makes me think of this question...Do the proponents of a One Buck Limit (OBL) envision that any given hunter will NOT be able to then take down multiple bucks while party hunting during a gun season, or not? I don't know the answer to that...but it could be a key differentiator. I would definitely agree that the vast majority of young bucks that are harvested nowadays are by young, or new, hunters OR by a random lucky dude on a gun season deer drive. Freezer fillers, if you will. I too have been in various meat lockers after a couple of days of gun season and there are often A LOT of young bucks there then. It really isn't something that I have seen during other seasons, FWIW. (Are there any stats as to WHEN the bucks are harvested each year?)
My understanding is that multiple bucks being harvested by any one hunter, while party hunting, which is legal, are NOT "showing up in the box score", because they are being reported by the tag holder, not necessarily by the shooter. I suspect that there are significantly more "multiple buck harvesters" people "hidden" by this wrinkle each year than there are dudes who are shooting two or more bucks per year and reporting their own deer on their own tags. Just my opinion on that...I know of no data that supports that. But over many years...I have witnessed that (multiple bucks via party hunting) than guys taking two or more deer per year on discreet tags.