sags from the outset

This mattress arrived saggy in the middle and firmer (in a good way) on the sides. I initially avoided the middle third of the mattress, and tried keeping to the sides until I realized how dumb this was. And then the one side I'd been predominately sleeping on started to soften considerably after just a few months. I know Nest is known for this mattress, but they changed the types of coils they use a couple of years back, and this has, in my humble opinion, done something really bad to the mattress.