Sure, I agree with you, but then all you need to do is lower the gap. The player I showed was just to point out how you can lower the salary of a player by basically 100k if you don't train any Rebounding. I understand that he gives up extra rebounds and due to that extra points to the other team, but all that saved salary also allows you to upgrade your other positions, that was the point I was making.
I still don't think you need to have 17-18 REB ever, unless you truly don't care about salary at all. In D.I I'm sure you can lower the gap, have bigs with 14-15 REB and they would still do fine in my eyes. There are so many NT bigs that have 15 REB around the world who dominate.
I think (for USA at least) :
11-12 REB is enough at the bigs spots in D.III
13 REB is enough at the bigs spots in D.II
14-15 REB is enough at the bigs spots in D.I
Murray/Harris/MPJ/Grant/Jokic - 2020 NBA Champs