Guptill is being persevered with because in 2013 in England he touched heights none of the other options are capable of.
Not sure the lineup you proposed gives six bowlers? Can’t expect Anderson and Ryder to bowl ten each.
Mills gets selected because he has fashioned a fine record in ODIs over 13 years, much better than many people give him credit for. He’s probably coming to the end of his time, but still has something to offer the current team.