The lethality of the nanobots (spoilers)

Not sure how necessary spoiler warnings are at this point in time, as everyone who hasn't been living under a rock knows how this movie ended, but...

Q is very explicit to Bond about how the nanobots are permanent. If you got them on you, that's it - nothing to be done. Like ever, ever, ever.

How can he be so sure that technology can't be countered by technology at some point in future? Imagine having your computer infected by a virus, and then being told "sorry, once the virus is in your computer, there is nothing to be done. Not now, not EVER. It is absolutely impossible for any sort of software to be developed which could deal with this, even in a million billion years."

It sounds like the writers had to think of a reason for Bond to sacrifice himself, and this is the best they could come up with. It would have been far better if Bond instead had to choose between opening the hatches for the incoming missiles, or escape - that he would not have time for both. But nooo, let's go with invincible technology.