(Let me emphasize again how much I'm generalizing. I'm not trying to be academic about it, just give you an idea as to my reasoning. You can see similar cycles in dynastic history - for instance, in China - and amazingly, this is also covered at length in the Twelve Kingdoms novel series, where one monarch basically equals a dynasty when you look at it this way. You see it in the anime too, but they don't spend as much time talking about it.)
So let's look at Tellius. Crimea and Daein have been around a few hundred years, and Daein has experienced some turmoil in the form of regimes changing (with rulers like Ashnard and Micaiah sort of symbolizing different ideals from the traditional monarchy it started out as), but they're both apparently having golden ages at the end of RD. Begnion, on the other hand, has been ruled by the same regime for eight hundred years - or if you want to get technical, probably only six hundred or so, as it sounds like the senate cropped up a bit after Altina.
But the senate has grown so corrupt, with the power in only a few select hands, that the corruption has probably trickled downward in the ranks. Consider the slave rings in PoR, and the fact that Sanaki had to manipulate Ike and Co. into taking care of them, contacting the Laguz Alliance, and helping the herons because she couldn't openly challenge them - because she couldn't trust her own officials to carry out her orders. She hired neutral, unaffiliated mercenaries to do the job instead.
At the end of RD - again, working from memory - Sanaki's ending script says she 'elected to lead her people without the goddess' and united the continent with treaties. The Japanese was a little darker as I remember it, but my translations are doomed to be imperfect; it said something to the effect that Sanaki united the continent through her efforts (treaties = fine interpretation) and then said something I interpreted differently than the official translation
What, no golden age for Sanaki? No credit for telling her people the truth - how else could she rule without the goddess? - and turning her back on tradition? No resurgence of art (maybe Oliver takes care of this) and philosophy? It's interesting, because it looks like Sanaki is popular enough to hold the country together just by sitting on the throne. That's the implication in some of her base conversations - that she's a good empress because her people were willing to die by the hundreds to put her back on the throne.
In a country ruled by the dreams of a goddess for six hundred years, I think it's probably just as reasonable to say that Sanaki has to replace the goddess with something her people can hold onto - but instead of looking outside of herself, she reached inward and became their pillar of strength.
Or-- when she dies, the empire will crumble. Lehran's little epilogue kind of hints at that. Begnion is declining. There may be a revival with Sanaki, but it's inevitable, I think. I also kind of like the idea she's the only thing holding it together. Is that my bias talking?
Why do these get so long? I'm better off writing fic, but speculating on this would demand an epic.