The scale will be massive, but just like the Nazi’s and others like them throughout history there is little choice.
The reality is about 1/3 of Muslims are fundamentalists, the other 2/3 probably would leave the relgion on the ash heap of history without much effort if they weren’t afraid they would have their heads cut off by the other 1/3 for it.
So yes, it is not going to be pretty, humanity is going to have to march through hell collectively to put this rabid dog down, but until it is willing to be honest and do it, this insanity will continue.
You cannot “moderate” Islam, there is nothing moderate about it... it cannot be saved from itself, the only options are destroy it, or continue to have it try to destroy everything else.
Playing nice with it will never work, no more than Chamberlains appeasement to Hitler prevented WWII.
How do you know the 2/3 would leave Islam if you kill 1/3? (Not to mention how do you know you are killing the precise right members of the 1/3?)
You would have to kill all 1.6 billion .... and even then the religion might attract others later to join if there is any trace left of it in history.
The idea does not seem practical. Even in WWII we did not kill every single German (and too, there are Nazis even today).