Ah, boosting stuff is a joke. Forrest may not become more than what he is, sure. That is as likely as Butler never improving as an overall player too.
They may be different players, Mann has developed a mean streak and that may be more of a character trait more than talent or whatever but their starting point is not that different from each other.
The discourse on Simmons is entirely different and complicated. Philly had other issues as well, I am not keen on discussing it much. The thing is others may pick up the slack for a 3-4 minutes while Forrest is on the floor. That is manageable. You can rely on our other guards for a few mins.
The development is rarely linear. You get better, you fail to implement new skills and look worse trying those. So looking bad lately doesn't mean you have to give up on a player.
I get that Butler has potential to be starting PG or a first combo guard off the bench. I don't think he sucks or anything and I wish we saw him more. But Jazz are already implementing new defensive schemes and the team needs to get used to that as well and Forrest is more of an immediate help and also has potential. Similar or worse defensive players like Shaq Harrison or Dunn come with their own offensive issues as well and I don't see a clear upgrade on the market.
I don't buy into the narrative that Quin is safe.