First of all, it's only natural that once golf became a big money professional sport, it's only natural that the competition for high paying positions on the PGA Tour would increase dramatically.
Many years ago when I was in college, I went to play pickup basketball, and ended up in a game with an All-American OG at my school, who is now in the College Football Hall of Fame. To my shock, he was just AWFUL; no hand-eye coordination, no shooting skills, nothing. If you'd seen him play, he would have been the last person in the gym that you would have picked to have an NFL career ahead of him. But he did because his skill set was suited to that, despite his lack of basketball talent.
I'll offer a counterexample to A.G.'s experience. I grew up in Palo Alto, CA. My Dad worked at Stanford University, and for a few years we had access to the Stanford basketball gyms, which we used regularly for pickup games at lunchtime. You would see some seriously gifted athletes at Stanford. One day in 1976, the competition had dwindled, and we were playing the last games of the day after 1:00 PM, and I got matched up against James Lofton, who went on to a Hall of Fame career in the NFL. He was also the NCAA long jump champion in 1978. At the time, I was probably 17 years old and a devoted basketball player. Lofton was probably 19, about two inches taller than me, and not very skilled at basketball. He could shoot a little, but it didn't matter. He was so fast and quick I couldn't do much on offense. He was big enough to score a few baskets against me as well. I was a pretty fast runner, but he was just so big and fast. He was quiet but nice, while taking some amusement in shutting me down. Very humbling.
Basketball and football have similar skill sets. If you are big, fast and quick witted, you have a big advantage. In basketball you can't be shy about physical contact. To be a football player, the desire for physical confrontation must be considerably greater. I think baseball and hockey have skill sets that are more closely aligned with golf skills, but all these sports have overlapping skill sets. It helps to be big and strong, and it helps if you like to battle and compete. Most important is a love and desire to play the game. It's common for an excellent athlete to be good at many sports, but not at a professional level, because it takes so much time and dedication to get to the top. It's no surprise that the two baseball/football combo athletes, Deion Sanders and Bo Jackson, possessed extraordinary speed.
I have trouble seeing Dustin Johnson, Brooks Koepka or pretty much any other pro golfer succeeding in baseball, basketball or football. Definitely not basketball or football. The chances that Koepka or Johnson could run fast enough for their size is remote, 1-1000 or more. One last random thought: I'm under the impression that having superior eyesight is a necessary trait for a baseball hitter, that having 20/15 or even 20/12 vision is common in the big leagues.
https://www.smartvisionlabs.com/blog/mlb-players-2020-vision-just-isnt-enough/Jeff Schley,
I can't remember what the exact numbers were, but I think you suggested that an athlete needed 4.5 speed to play linebacker or safety in D-1 football. That sounds a bit fast to me, since 4.5 speed seems about typical for fast NFL linebackers. My sense is you are exaggerating the required speed slightly, but maybe I'm wrong.