If 2015 is the big turnaround, they are targeting proven drivers like Alonso, they wont keep Button or they would have snapped him up longterm by now. Therefore 2014 is by far the best chance to trial young drivers. Why not have Perez lead Kevin in this 'gap' year. Grosjean has come good, Perez + Kevin would not have results far from Button + Perez in a transition year where the chances are the car will be weaker than this year.
Based on what evidence?
Based on the evidence that the car will get worse before it gets better. Based on the team have lost talent and are only looking to replace them in 2015, they are also using an engine for 1 year only before switching to a brand new engine and therefore design and packaging philosophy
How can they improve on 2013 in 2014 with those salient facts? The same group of individuals who have produced McLarens statistically worst season ever
http://bleacherreport.com/articles/1804295-statistics-on-mclarens-season-is-it-the-teams-worst-in-f1 are not changing till 2015. There is already evidence they have weight issues for 2014. Therefore the team would be smart to put all efforts towards a rebirth in 2015, and not something stupid like they did this year - redesign for 1 season only.
So best to blood the young talent now, Perez could have scored the points Button did this season if Button had not been there, he has been right behind him, apart from team strategies and team orders and a few exuberancies. If he became team leader I have no doubt he would score as many points as Button would