The universe Spock and Nero came from would continue without them, after all existence was around long before the both of them were on the scene.
Romulans are an off-shoot of the Vulcans, so technically Nero destroyed their ancestral homeworld, whilst ignoring the one he originally lost (seriously, if he knows what star will cause the destruction of Romulus, why not use the red matter on it and save it in the future, rather than go on a quest for revenge which was ultimately useless?).
The Romulan Empire would continue, with a new seat of power on another world. They would be shaken and weakened by the loss of Romulus, but it wouldn't be the end of them.