Silva, Aguero, Kompay, Nasri and Dzeko all have signed long term contracts. Kolorov has a 3 year deal as well. I don't think this a coincidence, but a plan by Txiki and Soriano. These players are in their mid to late 20's and by the end of their contracts will be in their 30's. Younger players will be bought in the coming years just like Mangala. All players age and someday they will no longer be a good as their old self. The young blood will be blended into the team and these older players can become quality backups when the time comes.
The 5 year contracts is also a way to deal with FFP. We don't constantly need to worry about paying transfer fees needlessly on players every transfer window knowing we have them tied down. We know we're set in these positions for a while and just concentrate on getting the younger ones in for now. If any club wants to come in and get one of our players, they'll be looking at a hefty transfer fee as well as taking on their long term contract.