I have to zoom to 200% to see noise in the sky (on a 216ppi 4k monitor). That's like looking at a 40" print. It's also only 15mp but it's from a 30mp sensor. Is it the cropped center?
If so then I would expect blue sky noise from ANY camera given this exposure and magnification. That noise could have been easily dealt with using NR in ACR or Lightroom, without destroying any detail.