So Romulus is an interstellar great power... which has reached the political and economic maturity of 18th century France? Farming, smithing, cobbling, weaving?
Sure. There's not inherent correlation between "political maturity" and being powerful. In point of fact, the entire concept of "political maturity" is very suspect -- it's based on the presumption that some cultures are superior to others, are more "mature" than others, and that cultures develop traits in a linear fashion as does a human being in his/her lifecycle. It's absolute nonsense, as we can tell just from looking at American history.
During the Civil War and during Reconstruction, the United States was in a phase of racial idealism. The 13th and 14th Amendments were passed, racist propaganda was actively being fought against, Republicans who believed in racial equality had control of the Congress and most of the state governments, African-Americans were being integrated into the European-American social structure.
And then the Democrats returned to power in the South, and later in Washington. The culture of European-Americans changed; Northern and Southern white folk forged an unofficial agreement that they were both just as American as the other, and turned on African-Americans. Thus began the nadir of American race relations, lasting from 1890 to around 1940. Segregation, Jim Crow, horrific bigotry, the works. John Brown went from being seen as either a hero or, at the very least, a David taking on the Goliath of slavery and losing, to being depicted as actively insane, irrational, and murderous
From the point of view that holds that there's a such thing as linear cultural development towards some sort of egalitarian, democratic "maturity," U.S. culture
regressed. But even that's absolute nonsense. It simply changed its value system to one incompatible with what it had previously held. And then, after the 1950s, it changed its value system
again.
There's no such thing as political "maturity." There's just change.
Tal'Aura would need at least to rely on the military, a patriotic elite, in order to take or retain power. I find it difficult to believe the average Romulan warbird commander or senatorial hanger-on--as opposed to the average haberdasher, tanner, shepherd or hunter-gatherer

--would think it's cool that she offed the entire previous government to which many would have personal and professional connections.
Yeah, but let's not forget that NEM established that a significant faction of the Imperial Fleet -- if not the entire Imperial Fleet leadership -- supported Shinzon in his coup, too. The implication seemed to be that it was the culmination two of long-simmering conflicts -- between Hiren's government and the Remans (who were, presumably, economically and socially empowered more than they'd ever been before the Dominion War), and between Hiren's government and the anti-Federation expansionists in the Imperial Fleet. The Remans and the anti-Federation expansionists joined together.
We know Romulus is not a democratic society. It's a dictatorship. We have no evidence that its people are enfranchised.
To be fair, we don't know how the makeup of the Romulan Senate is determined -- only that the Praetor is normally determined by the members of the Continuing Committee (established in DS9's "
Inter Arma Enim Silent Leges").
We do know from "Unification" that Senator Pardek represented the "Krocton Segment" and was considered a man of the people. It's possible that the Romulan Senate is determined democratically, or semi-democratically. Perhaps nobles inherit Senate seats and commoners are elected? Maybe commoners are enfranchised, but only if they own property or are members of the Imperial cult?
Given the makeup of the Romulan government that's been established in the novels and canon -- a mostly-ceremonial monarch (the Emperor), a head of government (the Praetor) chosen by a sub-committee of the legislature (the Continuing Committee), a large and centralized legislature (the Senate), a politically active aristocracy (various novels, including
The Art of the Impossible), a politically influential military (NEM), a secret police (the Tal Shiar) that's widely feared by the populace
and the military ("Face of the Enemy"), and a general population that's fearful of being accused of disloyalty ("Unification")... my impulse would be to compare it to post-Restoration England or Britain of the 18th and 19th Centuries before the Great Reform Act 1832. A hodge-podge and mixture of systems that's not exactly dictatorial but not exactly democratic. Illiberal, certainly, but neither autocratic nor democratic.
I'd feel most comfortable characterizing the Romulan Star Empire -- pre-Shinzon, anyway -- as an illiberal semi-constitutional oligarchic monarchy. (Enough qualifiers in there?)
On the other hand, one can make the case that if an empire is too centralized, if power is too concentrated in the metropolis, then the downfall of the metropolis could mean the dissolution of the empire as a political entity. Indeed, isn't that pretty much normal for empires? When Tenochtitlan fell, the Aztec Empire was done for. When Constantinople fell, the Byzantine Empire was no more (although by that point it barely existed anyway).
I'd infer that to be true of the Klingon and Romulan empires.
Ironically, Donatra's establishment of the Imperial Romulan State with a capital well away from Romulus may be the thing that ensures the continuation of Romulan society after Romulus's destruction!