Having no growls would be completely contradictory to what Opeth's signature sound is. I know, you can make the argument that there is no specific sound for prog since it's all about change, progressing and moving forward with new ideas. But with Opeth, the growls represent something much more than sheer brutality and anger much like most death metal bands are going for. The growls in Opeth (at least to me) are a representation of the ugly, dark and evil tones of their music while the clean vocals act as the beautiful, sad and often melancholic side of their music. To me, an Opeth album without any growls would be missing a large portion of their dimension and substance.
This is why Opeth remains my favourite band to play both progressive and death metal music. They aren't just using growls because "lol we play hevy mehtul".