He should definitely be kept if possible. Buffon, Lichst and Asamoah leaving (and possibly several others) mean a gaping hole in the leadership/example department. The whole reason United/Milan/Inter have been in such a huge mess in recent years is due to the ageing core leaving all at once and being replaced by foreign mercenaries who have no sense of unity and belonging to the club. Players like Mandzu, Marchisio, Barza are not necessarily the best players anymore, but we need to keep them to enhance the level of the rest of the squad.
However, if he wants to leave, we should definitely cater to his wishes (which I'm sure the board will do anyway).