There's no reason to sign a player who at best is marginally better than Forrest and interrupt Forrest's development as a smart steady backup PG.
Forrest has been very good lately. He doesnt do anything flashy, but he always makes the right pass and is good enough to put pressure on the rim while having that aforementioned ability. He's got the ability to make others around him better despite not being all that great himself.
Dunn doesnt have that. I will give you he's a more impactful man defender, certainly more aggressive, but Forrest fits the offense much better. The reality is we arent putting Dunn into a playoff game because he's going to entirely **** the offensive flow up. He's a bad bad bad offensive player on all fronts.