That is a rather short lifespan especially considering the costs per vehicle, I cant see them being as cheap as an ordinary low floor bus, but then they were nothing more than a vanity project anyway.
Why does London need a special bus anyway?
You weren't expecting today's plastic buses to last much longer than that, were you?

Even Routemasters weren't designed to last 40 years.
It's a total waste of money. London does not need another specially-designed bus to attract visitors. London's iconic image is the red bus, which all of them these days are. The image of the Routemaster, most importantly the halfcab, cannot be replaced by the lame attempt at the Borismaster. Open-rear platform is no longer compatible with the litigation-filled mentality of our society these days either. One day someone will take TfL to court because they injured themselves when jumping off the open platform. Mark my words.
It is a project to satisfy some people's egos. No more, no less. All that money wasted in running this NBFL project could have be invested in converting more conventional diesel buses into "clean" ones or simply buying extra "clean" buses.