I agree. Letting Sanchez walk is an easy decision. But in a couple years, when the Yankees are paying close to $90-100 mill to Cole, Stanton, Judge, and Torres (plus another $21mill to Severino and Hicks), building the rest of a competitive team around them will be very difficult.
Stanton can opt out after this season, but he will be 30 years old (when free agency stops being kind) and walking away from 8 years / $212 million. And he won't get either of those numbers as a free agent, especially since he has only topped 4.3 fWAR one time since 2014. Even if he wants to opt out, his agent will absolutely advise against it. And he did not hire his agent to ignore his advice...