Please stay Pirlo.
Yes, Pirlo's transfer could potentially give Milan one of the best deal of this summer's transfer market. By selling him Milan could still get a reasonable amount for him, plus even an extra player should Chelsea be willing to part with one of their players.
Ancelotti would probably stop at nothing to make sure he gets his focal player at Chelsea, even if it means dishing out a few extra million or a player that hasn't quite past his day and would fit Milan's age requirements (30 years or more). If it's just money Milan get this will open up for some potential transfers but how many players can 16 mill by you nowadays when the team needs a huge overhaul?
And yes, he isn't as fast as he used to be and his style of play is being read more and more easily nowadays leaving an element of vulnerability. But this is Pirlo, the shining star that can create nothing out of something, and quite frankly Milan's only creative player at this point in time.
Seedorf has mastered the word inconsistency, while Di Gennaro isn't guaranteed a starting place and will have to work his way up. Pato has said that he wants to return to his deeper role and not continue playing the traditional 'centre forward' position but Milan have not made any signings to allow him the space he needs.
Not to mention the roughness of the Premier League would never gel for Pirlo's delicate and intricate style of play. The fast pace of the Brits will not allow Pirlo to have as much freedom on the ball as he has put to use so well in Italy. It will be the toughest battle of the "Tinkerbell's" (as he is known by Milan fans) career.
This World Cup Winner is still one of my favourite Milan players and I feel with Leonardo at the reigns he still has a lot to offer us. The Brazilian can try and rework Pirlo into a position that will leave him free on the field.