Has muscle mass been shown as a predictor of long life expectancy

Yes. Greater muscle mass—and particularly greater muscle strength—has been associated with lower all-cause mortality, though the association is stronger for strength than for muscle mass. 1

• People with low muscle mass or strength face higher mortality risk: In the comparison described, low muscle mass was associated with about three times the all-cause mortality risk of high muscle mass; low strength was associated with an even larger difference. 1
• Strength shows the stronger association: The discussion notes that the muscle-mass finding may reflect its close association with strength, and estimates a roughly 3.5-fold hazard ratio for low versus high strength. 2
• Strength training supports function with age: It was described as a way to combat neuromuscular aging and preserve strength and power needed for movement and catching oneself from a fall. 3
• Other longevity-linked measures: Handgrip strength and lean mass were described as inversely associated with mortality, with the association becoming stronger at older ages. 4