Kevin Mayer of France is the current decathlon world record holder. If we look at the marks he scored when he set the record we see that: 1. He would have been worse than the top US high schools boys in 2021 in five events (100m, HJ, 400m, 110h, and discuss); 2. He would have been better than the top HS boys marks, but well off the top collegiate marks, in 4 events (LJ, SP, PV, and Jav); and, 3. He would have been slower than the top HS GIRLS in the 1500m.
In other words, the best decathlete ever would not be able to beat HS athletes (including girls, in one instance) in most of his events.