Cousins is a loser. He has bad body language and hasn't really made his team any better. I think he's talented as heck but I think he's a crappie teammate and leader. I also don't think know he fits the mold of what Ainge and Stevens look for: high character and effort guys. I think it's likely they keep the pick and go for someone like Horford in FA.
Does Simmons fit that mold?
I'd take my chances on a 19 year old maturing. Cousins is 26. You kind of know what you get at that point.
Cousins is 25, but sure. You know you're getting a top-5 scorer and rebounder with a bad temper.
In Simmons, is there even a 5% chance he becomes as good as Cousins? He may mature, or he may become the next Royce White. Either way, nothing about him fits the "mold" you were advocating. To date, he's a low effort, low IQ loser of a player who couldn't lift his team to the tournament.
We know Cousins can dominate. We know he has a huge impact on his team's point differential (#12 in RPM). We know none of that for any potential draftee.