I don't think it is a "no-brainer that Richard Marshall has to be released at this point of the offseason, or at all. A $5.8 million cap hit for Marshall is high for this upcoming season, I agree with that, but that was also part of how the free agent market works. Players always get overpaid to be able to secure their rights. It is why a guy like Mike Wallace is going to be getting a $10+ million a year contract when his playing worth doesn't match that cost, just how free agency works. The Dolphins currently have $44 million in cap space and a cap savings of only $3.4 million at this point is not really needed.
Marshall is an above average cornerback and he had some respectable seasons with the Cardinals and Panthers before signing here. He got hurt here last season, that happens to players during their career. It wasn't like the Dolphins signed an injury plagued cornerback since he hadn't missed a game his previous 7 NFL seasons. If Smith doesn't re-sign or get the franchise tag and you cut Marshall to save $3.4 million, that leaves the Dolphins with Dimitri Patterson and Nolan Carroll as the top two cornerbacks. Granted the Dolphins can sign and draft replacements, but why create another hole just to save a few million when the team already is loaded with space? If you sign free agent cornerbacks and draft some who can replace Marshall than you can make a decision to cut or trade him.