Sheldon is the only one who will be traded for a 4th or a 5th. I believe we should keep Decker and Marshall simply because we have no QB that we know of. Everyone else is pretty much a cut. Maybe restructure Mangold if he thinks he can last a couple more years.
I think we need to keep Sheldon since his stock is so low but not unless we can sign him to a somewhat reasonable extension. We should also invest cap space and / or a draft pick on an OLB so we can move Sheldon back to DE. This gives us depth in case of injury, the opportunity to rotate DEs to keep them fresh, and some leverage when we have to extend (or trade) Leonard Williams when his rookie contract is up. Also, we could potentially trade Mo at the tail end of his contract if we have Sheldon and Leonard locked up at that point. Losing a first round talent (Richardson or Pryor) for a third or fourth rounder is how we stay mired in the bottom half of the league.
I've always been of the thought that the way Mac structured Mo's contract with a get out option at the end of the 2017 season was so he had the flexibility to make a decision on Mo vs Sheldon. That's why I dont expect us to trade him now, but if someone blows you away with an offer you have to take it.