generators, etc....

Sorry wrong answer. Five 9s is less than 6 minutes of downtime/year.
A large amount of the telco gear is only rated at six nines.
(Six nines is less than 1 minute of downtime/year.) Thats not
even getting into the issue of downtime v. availability.

With the delays in BGP processing commonly seen today, (on the order
of 15 minutes), one prefix flapping would put that prefix at
less than four nines (11 minutes).

Three nines, (just under two hours), would be a good goal for
planned availability. Then of course, there is the issue of
maintence. I understand telephone switch software designers
have the problem of all switch upgrades have to be done with
the switch running. There is a whole programming group at
some companies that searchs for extra bits in structures to add
new features, because they can't reboot the things to change
the size of data strucutres...

That just ain't gonna happen, when your favorite router vendor's
best guess for fixing some problems is to reload the router....
(or you have to install new firmware, and do a reload to get it going...)
Not all routers may be this way, but for any deployed networks
today, three nines is going to be an upper limit.