I say give him one more season and then move him on.
He's on £300,000 per week, his best days are behind him and they have much better alternatives, both up front and in midfield. His legs went a few years ago and his form has never really recovered since. He was in his peak around 2008-2012 and after that has been progressively more disappointing each season.
He doesn't seem to play with the aggression he had when he was a bit younger either, and that was a huge part of what made him so successful. It seems he's physically and mentally diminished as a player over the past few seasons. He's played about 40 games a season since he was 16 though, so I'm not surprised he's winding down a few years earlier than most other forwards in his age group.
He's still got a couple of years left in him, but I don't think they should be at United. He just doesn't seem to fit there anymore. He's been a fantastic servant to the club but I think his time at United is running out.