The thing is , as we are practically in an oil crisis right now, there will probably be no civil aviation as we know it in 20 years time..... so what will it matter ?
With oil being burned at rates practically beyond production ,and talk of the 'peak' being reached why worry about airliner lifespan ?
Unless ALL countries (and one in particular that burns petrol as if it were water) dont start making a REAL effort into efficiency and conservation instead of gratuitous excessive use, then there will be no fuel to put in these new generation jets.............