I think the reasoning is that as he has got older he may benefit from batting at 3 whilst also being able to concentrate on the captaincy. I am no expert but I would be interested in seeing his averages for the first 75% of his international career and then for the final 25% and see if there is any variation / decline to warrant moving him down the order.
im not sure whether Cook wants to carry on skippering, but as a batsman he could, form permitting and presuming he does not want to go off and work on the farm, play for another....4 or 5 years?
Gooch did it and he's very much in the same mould....
whether he still has the desire and is batting well enough in a year or so's time no one knows.
but potentially he could play for quite a while yet.....
personally I think he will play as a batsman only after next years Ashes