Not necessarily true.
- Utah: didn't guard Markkanen or Clarkson
- Lakers: didn't guard Lebron or AD
- Portland: didn't guard Grant nor Sharpe
- Mavs: didn't guard Luka or Kyrie
Keegan has shown a lot more defensive versatility especially with stepping out in the perimeter, but he has not been tasked with guarding the best players on the opposing teams. He has not played at an elite level on either end of the floor. I think Keegan has made a clear jump on defense, but I'm not sold that it's as good as people are claiming. 13pts 6rebs 2asts and 1stl is not a superstar level nor anywhere near HOF.
I understand you're making future projections down the line, but he hasn't been able to be elite on any one side of the floor, let alone both sides. You're also admitting yourself that his defensive improvement has come at the expense of him shooting poorly. His jump in defense are not worth him shooting just 30% from 3pt. You bring up superstars and HOFers, but scoring is what makes them great above all else.
This team doesn't need a Cameron Johnson. We need Paul George. None of those 2-way stars (Kawhi, Butler, George, Lebron), have never scarified offense for defense, but they've all been better natural scorers.
An elite wing is the difference between fringe playoffs and contending teams. We need him to make that offensive jump more than being able to guard 35-year-old Steph in 1 game.