First, I think you’re underestimating how difficult it is to acquire a good, starting caliber 3&D wing in this league that justifies supplanting DeRozan in the starting lineup. Which team wants to give up said player? And what are we paying for said player?
Second, I think you’re underestimating just how valuable and impactful Ellis is. We already have one of the most elite roleplaying defenders in the league. Why not optimally use him in our roster construction and rotations? Here's a quick analysis I did across 3 advanced impact stats to help illustrate how rare Ellis impact and productivity is...
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So when looking at only guards/wings/forwards, Ellis is in the top 8-15 in terms of defensive impact, and among those elite guard/wing/forward defenders, only 1-2 of them are more impactful offensively without being a star/go-to option.
Keon Ellis may well be one of the very best 3&D, roleplaying starters in the league. He needs to be starting and closing out games for us. And considering he's a terrific fit at SG next to either Fox or Monk at PG, I think it would make wayyy more sense to move either Fox or Monk for an upgrade elsewhere (or to somehow convince Monk to accept that he will not be starting or closing out games anymore).