He's actually not retiring but 'moving upstairs' and is going to be called Technical Director and still has overall charge of all Crewe's teams and their academy. The person who will effectively be manager is Steve Holland - his title will be first team coach. The changes are from the 1st July.
There's been quite a sizeable chunk of the Crewe fans (which doesn't amount to many people I know) calling for Dario's head at various points during this season and there was a lot of trouble at the club's AGM when some fans tried to put through a motion of no confidence in Dario (it was allowed to be discussed).
Obviously he is generally well regarded though and I think most of their fans think of him as some sort of saint but recognise it was time he moved aside.