There's also the deflector in the eject chamberso the gas generator pressurizes the chamber rather than blasts the nozzle.
Bringing this back around ot the OP, the climax of The Spy Who Loved Me when they cringed at the screen that looked like the two missiles were going to collide in space. It's bad enough that every representation of a an ICBM or SLBM missile in flight stays intact from launch to impact. Apparently the third stage motor just burn through the first and second stages when it's its turn... and it's powered flight all the way to detonation....
And where are the fires of re-entry??