While I certainly have my issues with the Kelvin movies, I think you are sorely underestimating the logistics behind having to evacuate a planet whose population is in the billions in the amount of time that the Romulans had.
One must also take into account the fact that the Romulan empire, as well as the Klingons and the Federation, were less than a decade away from the end of a very costly war with the Dominion, that would have seriously depleted ships and resources.
Would the ships they had left be capable of mounting such an operation? Did they have the capability of building enough ships capable of evacuating entire populations in the time they had left? I'm thinking of the real world parallel where the United States was able to overwhelm Axis powers through sheer industrial capacity in WWII when tanks were practically being built on assembly lines.
Later canon (yes, "canon". Deal with it.) shows that Starfleet had a ship specifically designed for evacuations of this sort - the Wallenberg class. Also, Federation starship production at Utopia Planitia would be assisted by replication technology to the point where the ships are almost put together by themselves. (I mark this down to improvements made by Commander Shelby when she said that her task force would "get the fleet back up in less than a year".) Did the Romulans have industrial capacity of that magnitude?
Also, and finally, the Romulans were hardly "wiped out" in that event. While there were certainly displaced refugees, and despite the abolishment of the neutral zone to accommodate them, the Empire itself merely reorganized into the Romulan Free State. The Federation itself still honors all of its treaties with the former Empire, so you can't say that it isn't a continuation of governmental structure in all but name. The Romulans were certainly large and relevant enough of a faction to be considered equal partners on the reunified Ni'var.
... bash new Trek.