It is often said that you cannot win them all and if you apply that logic to free agency then you cannot bring them all back. We have seen the Cowboys make a few emotional yet difficult decisions since the new league year began. The cover move for that book was undoubtedly releasing Ezekiel Elliott, but the Cowboys also let players like Dalton Schultz and Noah Brown walk (both incidentally to the Houston Texans) which could not have been easy given that they came up through the Cowboys system.
That the Cowboys have acted unemotionally so far in free agency has been what people have praised them for, and that disposition is part of what leads to decisions to not bring certain players back.