I think it partially depends on how we interpret his troubles. Were they partially based on an adjustment period where he was getting used to being the third option and going for long spells where he was merely a spot-up perimeter jump shooter? Or were they because bigger, more athletic players were man-handling him defensively? I tend to think it was more the first option, though we can't entirely dismiss the second, as his best series came against the soft Lakers defense.
Still, I think there is definitely the possibility he becomes more effective next year as he becomes more and more comfortable with his role. Better yet, if he continues to accept that Pierce and Garnett will get more touches than him, it will make things easier as he gets older.
All that said, I completely agree about this trade nonsense. Ainge may have said he would've traded Bird, but he didn't say that coming off winning a championship: he was referring to his decline right around the turn of the decade. If Allen goes through a similar decline, Ainge probably will trade him or let him go. However, until that happens, no trades should occur.