"maintain and secure"=protection against sabotage=beat any malicious third-party to it.
The dreaded 51% attack? That's not viable anyway. Fork off, kick out the responsible peers, resume mining on the 49% fork, 51% fork devalues, done. Whooppdeedoo.
Now if you want to kill bitcoins, there are way more viable ways to do it. Like cutting off the arms ears eyes and mouths. Which is coincidentally exactly what's been starting to happen these last few weeks.
Had Bitcoin not being designed this way, it would have been killed by a different kind of 51% attack in its early days, Bitcoin doesn't only need to face threats of governments, but also that from botnet operators, random script kiddles etc. In fact, that we are dealing with government pressure itself is evidence enough that the system worked. Also, 51% attackers can follow you wherever you go, however many times have hard-fork.