Hainsey was a better partner for Rielly, but I think it was clear, he wasn't going to be the right guy moving forward, age, and mileage. Better hockey player, as a standalone... I think I'd argue not, but for sure, a better partner for Rielly.
Gardiner was better, but post injury, hasn't been better. Plus the whole R/L handedness thing... Gardiner, nor Rielly have really excelled on the Right side at any point. Plus we needed the grit in Muzzin, meaning Gardiner as a third pairing, for the cost wasn't an option.
We needed to move on from Nazem, I think we all admit that, as much as I liked him.... and that's what we got in return.... again, we can speculate that we would have hoped for a better return, but we don't know if such an option existed... we got a 3C, and RHD... even if he didn't really fit our needs... (though his offense has been good since Keefe)