Seattle would be happy to keep those five starters as well.
The Mariners hope to improve on this year's team OPS+ of 102 and team ERA+ of 105 just as the Red Sox hope to improve on this year's team OPS+ of 105 and team ERA+ of 106. It's not as if Mariners or Red Sox are bottom-feeders by either measure.
Perhaps the Mariners should adjust their ballpark to help hitters (while necessarily hurting pitchers).
Free agent hitters almost always refuse to sign with Seattle. Since Jerry Dipoto's arrival nine years ago the largest guaranteed free agent contracts to hitters went to Mitch Garver (two years, $24 million last offseason), AJ Pollock (one year, $7 million the previous offseason) and Nori Aoki (one year, $5.5 million for 2016).
Free agent pitchers apparently find Seattle more attractive as Dipoto has negotiated more than $200 million in free agents contracts for pitchers, topped by the five-year, $115 million deal for Robbie Ray.
The Mariners are in a tough spot ... as are many MLB clubs.