You are wrong on both points. Way more than a dozen or so players who could challenge Nicklaus. There are not 60 or 70 great players now. Xander Schauffle (sp), Max Homa and Patrick Cantlay are currently in the top six. (They are very good but not great) There are lots of very good players and substantially more than in previous times, but not very many who could challenge Nicklaus or Woods [Please note that I have mentioned Woods in this discussion] on back nine of a major on the final day of the major.
Going through the people who won in 1972 her are some really good players. Tony Jacklin (international), Tom Weiskopf, Gary Player (international), Lee Trevino Dave Hill, Chi Chi Rodriguez, Bruce Devlin, Doug Sanders, Jim Colbert, JC Snead, Lou Graham, Lanny Wadkins, David Graham & George Archer. A partial list of those winning in 73 includes, Billy Casper, Bruce Crampton, Gene Littler, Dave Stockton, Johnny Miller, Ben Crenshaw and Hale Irwin. Some non-winners during those 2 years included, Tom Watson, Ray Floyd, Tom Kite. (I didn't research this closely) All of these players could compete on today's tour and do well.
Would add that I used to be a Palmer fan mainly, but Nicklaus won me over with skill and sportsmanship. Ohio ties have nothing to do with it, and for instance, I don't like the way he designs courses. From what I have heard his redesign screwed up OSU Scarlet.
The mistake you make is equating golf (which is at least half mental) with very physical sports like football and basketball. I can see how people can question whether Jim Brown would be as dominant now as he was in past because he wasn't as fast as the top runners today like Saquon Barkley. (maybe with training he would be. He was an extremely great athlete). Anyone who watched closely when Nicklaus and Tiger were playing knows that their mental toughness over long periods of time greatly exceeds that of any of today's players. Maybe someone will jump out of the pack, but so far it hasn't happened.
In any event, here is Nicklaus talking about his 3 best shots, all 1-irons, including a 242 yard 1 iron (almost certainly with a balata ball) at the 1975 Masters which he had to hit high to hold the green. Very few of today's golfers could hit that shot and even fewer could hit it on the back nine of the final round when it mattered most.