Personally I feel letting him go at seasons end would be a clear contradiction of everything this club stands for.

Heskey is a good guy, has a great accent and played at the highest level. He says he loves newy too, and foxsports seem to like him
He should be allowed to play on for as long he wants.
Next season he should be allowed to retire at a point where he can't be easily replaced.
Then come back when he feels up to it.
On field performance should count for nothing.
And at some point he should be involved in a row with a player of real quality and drive him out of the club.

Anything else would be unjets.