You're looking at this from a healthy body's perspective, in case of an excess of misfolded proteins the ER stress increases which activates the unfolded protein response (UPR) which tells the cell to produce less proteins to let it chill out a bit, and then if the ER stress remains too high leads to apoptosis (cell death). That's the way the body normally functions and it's all good.

But if your body produces a majority of misfolded proteins you don't want your cells to freak out about it and kill themselves because you need those cells alive for your body to perform its normal physiological functions. At least it's an emerging theory that in some case the systematic destruction of the cells leads to worse outcomes than just having cells living and working with misfolded proteins.

Take all this with a great big grain on salt because it's the blind leading the blind. I'm not a biologist, I've just read a lot about my illness.