If they had been a one used enemy, Romulus as a cut off world after Earth's victory might make sense. But is still would make the lack of knowledge about just who the Romulas are even more odd if Starfleet has been sitting right inside their home system for the last 100 years. Their sensors tend to have good enough range for that sort of thing.
Also the concept of the Nuetral Zone seem more like a draw happened, rather than a total victory one might expect to cut off Romulus from the galaxy. From the "known" galaxy would allow for a simple curved border that goes away eventually as it reaches unknown space. Though this also leaves the other side of the Star Empire "open"