Melky's best seasons came when he was 26 and 27 years old. Obviously we don't know for sure that he's done his peak, but based on the performance curve of an average player, his best seasons are likely behind him. The average players peak in their mid to late 20s.
That's just our best guess, and anyone's best guess really. It's not absolutely going to be true.
The fact that Melky had an excellent looking season where he basically maxed out his hitting abilities, tagging almost everything, and he was still only slightly above average in total value... doesn't really bode well for his future value IMO.