Theory : Dr. Primitive is not trying to make people into monkey, he is trying to make everyone into Leviathan.
I know, banger title. But hear me out.
First, let point out the elephant in the room, Ashveil.
Here's the question. Why, in the fight against Dr.Primitive, was Ashveil not affected by Monkey Beam ?
We are probably familiar with how Ashveil is super Voracity-coded right, but why ?
The timeline is not hard-canon ( typical Hoyo games ), but we know that the Zulo fight happened before the Dr.Primitive fight ? Since iirc they said after the latter one the Galaxy Ranger basically went into hidings.
From what we heard about Ashveil, his special hand already devoured Zulo and a part of Shuhu, which means he already got this power before Dr.Primitive.
So what am i trying to say here ? This Voracity power, is what gave him the immunity to Monkey-fication ?
This theory is working on the basis of another theory being true, which is the Leviathan's Conception Theory.
In the readable Species of the Galaxy: Wubbaboo , it has a part that said :
From this sentence, we can probably deduce that all current organic life form can be traced back to Leviathan's cell.
So maybe, making everyone into monkeys was just an incomplete process, the true goal is to make everyone into Leviathan, or at least, awakening its super ancient DNA inside everyone.
Also shout out to Lygus for saying this. I heard that in CN he used the word "Beast" here, which is the same word in the CN version of Leviathan, which is Ancient Beast.
As for why he's doing it, maybe to fight against one of the 4 Apocalypses as Lygus was mentioning here i think ?