I think there's no question that making an Olympic team or setting a world record is significantly harder in 2024 than it was in 1964, simply due to the financial incentives that exist today. It's hard to quantify how much that changes, but it definitely changes a lot. There are still athletes like Eric Holt who are reaching a very high level without a pro contract (although he still does get a fairly significant amount of prize money, but not enough to live on and not reliably). Collegiate athletes may be making money now but I don't think that's significantly changed the landscape for some athletes, so I imagine guys like Nico Young would still be running quite fast (maybe not 12:57/26:52 though, especially if the world records were over that). But on the flip side, there is little doubt in my mind in the amateur era Grant Fisher would be an engineer right now, and Yared Nuguse would be in med school.
So it's really hard to say for sure whether a guy like Kessler or Simmons might be considered world class if athletes were required to be amateurs. This is compounded by the fact that all the shoe technology, and even the fields in which they set their records, has all been influenced by the introduction of money into the sport. I agree that the best way to go about it is to simply separate the professional era from the amateur era, and in that lens I think regardless of shoe tech or track surface, 13:25 is still the best 5000 and one of the greatest accomplishments of any high school distance runner in the modern era (perhaps the greatest), and it is exciting to see what he will do next.