King Daniel wrote:
The Romulan Empire has been shown to be so centralized that offing a single room full of people was all it took for Shinzon to take over in Nemesis, and of course the aftermath of that was the RSE/IRS schism that lasted a couple of years. I'd say it's a possibility that the Romulan Empire will fall or splinter after it gets supernova'd. But of course, we shall see...
Well, it wasn't a room full of just anyone. It was the Praetor and most of the Senate. And just because the movie doesn't show it, it likely took months or years for Shinzon to gain enough backing from the Romulan military to be able to claim power.