Seems a bit unecessary to me.
Look, I hope he stays here for 4, 5 or even 20 years as this will mean that he's made a success of the job and that we've really become one of the best teams in Europe, but... what if he doesn't? What if, at the end of next season or the season after we've not progressed as we should? Success is not guaranteed and that is in no way a judgement on Pep's managerial abilities as I do belive he's a great manager, however sometimes things just don't work out.
Why not wait until the end of next season, when we can see how things are going, to give him a new contract? Why does it have to be the end of this one when we quite possibly may have had what could be viewed as a relatively disappointing season? If it's to "send a message" out as some people have suggested, I ask who to? The media? Why give a fuck what they think or say (the club usually doesn't). Players? Frankly anyone signing this Summer isn't going to give a shit whether Pep has 2 years or 3 years left on his contract.
As I said,I just don't see the need for it. If it doesn't work out the only thing that is going to happen is that it will cost us twice as much to dispense of his services.