The bad news is that I am really strapped for time right now, because I'm hosting several family members from around the country as we gather for a funeral. The good news is that I posted some material last month when you asked for foundational posts, that I think might translate well to twitter. In particular, the year-by-year analysis of Jack's alleged dominance in post #6430
could probably be trimmed down into one tweet for each year, especially if you remove my editorial snark. I've found that most golf fans are surprised to see how thin Jack's "dominance" was when you look at the Jack era year by year. He really had only one year that compared in dominance to any of seven years of Tiger's career. He had five years where he was clearly the best in the world (but not necessarily dominant), compared to 10 for Tiger. He only wins when you look at how many years he was arguably in the wold top ten -- 20 years to Tiger's 16 (so far). Being top ten in the world is very hard to do, but it's not domination.
And I am as puzzled as you are how Brandel can be so oblivious to the strength of the fields in the pre-1980's majors. You only have to look at the Open.com website to see that in the 11 years 1959-1969, when the "Big Three" won five British Opens, there were never more than a dozen Americans in the field, and if you eliminate the amateurs and seniors, seldom more than half a dozen (zero in 1959, when Player won his first Open). There were never more than 30 (including seniors and amateurs) all through the 1970's.
To be specific, look at 1968. The British Open paid $7200 to the winner. All but five regular PGA events paid over twice that much in 1968, and the Greater Milwaukee Open, played the same week as the British Open, paid the winner $40,000. In today's terms, that would be like having a regular PGA event with a first prize of 11 million dollars played the same week as the Open. There were 11 Americans in the field, but only six of any distinction, along with an amateur and four pros with a combined total of one career PGA win. And as inconvenient as trans-Atlantic travel was then, it wasn't nearly as tiring or expensive as travel to Britain from Australia or South Africa, which meant that even though the Open had far more prestige outside of the US than in it, it was missing a large percentage of international stars, as well as 90% of US stars.
The field of the 1968 PGA Championship was described by Jack himself as "absurd and unfortunate," with about 50 touring pros and about 110 club pros. Just about the only way a non-PGA member could get in was to win one of the other majors, so foreign players who didn't want to devote a year or more to the PGA Tour were effectively shut out.
The US Open was indeed open, but you had to qualify in the US a few weeks before the tournament, which meant that someone in Europe, South Africa, or Australia had to either commit to over a solid month of his time away from home, with no guarantee he would actually get to play, or make two overseas round trips in a month. As a result, only a handful of international players entered. All but one of Europe's leading money winners for the years 1955-1975 never played in either the US Open or the PGA Championship in their entire careers. The one exception, Peter Oosterhuis, never did it before 1975.
After discounting amateurs and seniors, the 1968 Masters had only about 60 players in the field. Credit to the ANGC, they did make an effort to invite international players, but the time and expense (plus the fact that majors weren't MAJORS then) caused many invitees to decline. Peter Alliss was one of the best players in Europe for nearly 20 years. He won the Order of Merit twice, and beat the biggest American stars like Palmer, Venturi, and Casper in his Ryder Cup matches, but he was invited to the Masters only five times, and he only accepted twice. Too far to travel, he said.
The result of all this is that the US Open was probably the strongest event of the Jack era, but with a virtually all-American field, it was only about half as strong as a full field major today. The PGA Championship, with two club pros for every touring pro, was probably no stronger than a regular PGA event of that era, and weaker than a regular PGA event of today. The Masters, with only 60 touring pros, was not much stronger. And the British Open, with just a handful of players from outside of Europe, was weaker than almost any regular tour event then, and much weaker than any of Tiger's official wins.
Since the GOAT debate though most of the 60's was between Snead and Hogan, neither of whom had as many majors as Hagen, there was nothing like the pressure of majors today. A major was especially nice to win, but it wasn't a life-changing event like it is today. "Most majors" wasn't the most important stat until around 1975, thanks largely to years of lobbying by Jack, the only man who played all four majors every year.
Note that this comparison doesn't depend on the fact that there is a much larger talent pool today, or that athletes in every sport have gotten much better than they were 50 years ago. Even if the players of the Jack era were as good as the players today, from the best in the world to the 100th in the world, it's still an incontrovertible fact that it was rare for half of them to show up for any given major.
Also note that the relative strength of the US versus the rest of the world doesn't matter. Today, the world golf rankings show a pretty even distribution of Americans and non-Americans in the world's top players. But suppose it was 90-10 in favor of Americans in the Jack era. That would make the US Open relatively stronger, but still nowhere near today's majors. It wouldn't help the Masters or PGA much, since there were still only about 50 Americans in their fields. And it would make the British Open even weaker.
The bottom line is, the reputations of the "Big Three" were built on winning majors with fields no stronger, and sometimes much weaker, than a regular PGA event today. Jack was less dominant, for fewer years, over weaker fields, than Tiger.