Being serious for a second
When we defeated nazi germany did we exterminate every single german?
No we didnt cause we realise no race is evil just the people who lead them plus we realised we needed them to help fight a bigger foe the USSR
and seeing that varian is now become a boy scout he will have no desire to see the orcs scoured from the earth instead try to form a working relationship with them and when the demons start appearing he will be thankful that the orcs are around to help save azeroth like they did during the third war