Well, I'd say 1) immutability is expensive[0] and 2) mutability is sometimes simpler and clearer than immutability for some algorithms IMO. So I'd want python to have both.
Today's passion for 'immutable everything' is going to burn out pretty quick as it's great but being used too much and without recognising its cost. IMO again.
[0] if naively implemented, and it so often is.