Cider, like any other wine, has an arc of quality, based on a complex relationship between, sugar, tannin, stabilizers, cleanliness, carbonation, acidity, containers, closure, environment etc etc. As a rule of thumbs, sweeter wines have shorter arcs. As do low-tannin and acid wines. Due to it's natural properties, cider tends to have a relatively shorter shelf life than, say, a dry red wine. Especially when kegged.
Sounds like your quality arc has peaked. Good excuse to make another batch