Quite a few things.
For starters, if Creation were not at all what it is, history might look quite different.
By which I mean, do you really want your entire culture to have a handful of Celestial gods acting as lynchpins that hold it together? Gods are gods, and always have been—they behave as well as they're forced to.
Putting it another way—Wyld-shaping technique was a premiere producer of the impossibilities the First Age ran on, but not the only one. You can also get, perhaps unsurprisingly, a lot of really powerful mojo from the most powerful denizens of Malfeas. Of course, no Solars means no Solar Circle sorcery, which means you have to bargain with the Unquestionable for their help keeping your infrastructure running.
You can probably identify some problems with making Creation's entire technological infrastructure, particularly its defense network, overly reliant on demons.
Moving on: Yes, you have Essence 7+ Sidereals to work with. Sidereals are chronically over-worked with top-priority business like 'make sure reality doesn't spontaneously melt.' This gets more and more true as they grow older; the Essence 7+ Sidereals have much more vital things they need to be doing than keeping the water purification plants in some city built on a toxic bog in the Southeast working at peak efficiency.
Okay, so now we're down to the Dragon-Blooded and here's where we start running into serious trouble. The Golem-Producing Wonder Factory uses square circles to calibrate its transmotic armor-sculpting armatures, which produces defensive automatons whose outer hull is both flexible and absolutely rigid, making them nimble and hardy fighters. Square circles break down after a while; their impossibility grinds against the mathematical certainties of Creation.
With the Solars gone, there's no reliable way to get more. So after a few years the Wonder Factory's automated production lines shut down. Okay, you're still left with your golem army, but every time the Fair Folk make a sortie against the region, a few of them get destroyed, so their numbers are dropping.
The Terrestrials examine the factory as best they're able. It's fucked; they can't fix it. But maybe they can make more golems by hand? They look into this, and yes, by hook and by crook, they can still hand-assemble these superlative battle golems… but it takes a skilled Terrestrial artisan and his assistants months and months to make a single one. That might slow the rate of loss, but a) you're tying up a technical expert and this is far from the only system experiencing difficulties and b) you're still losing them faster than you can make them.
So the Wonder Factory is closed and scrapped for spare parts. Eventually the golem army gets whittled down enough that it can't effectively protect the entire region; now individual local daimyos begin fighting one another about how to allocate the remaining golems, to best protect their holdings.
Things get worse, though. Every time the Fair Folk manage a breakthrough that eats away at the edges of Creation, that shrinks the world. There's nobody with Wyld-Shaping Technique to go get that land back, much less to actively expand the borders. Moreover, maintaining the land is hard—the Terrestrials are doing their best to uphold the left-over Solar edicts that make up the Order-Conferring Trade Pattern, but they're not up to actually understanding the system in its totality. When the raksha start punching holes in the pattern, the best they can do is make educated guesses about how to compensate and reroute trade networks. It doesn't always work properly, and so the stabilizing network that holds Creation's borders together slowly but steadily weakens.
A Skywolf-class aerial battle cruiser crashes during a battle with the 118th raksha air kraken division. Well, that's okay-that technology is fully within the understanding of the Shogunate, they can replace it, right? Well… yes, but. Gathering up the enormous amounts of required materials is hard, because most of the teleport gateways that were the apogee of Solar technology got broken in the Usurpation and nobody has been able to get any of them back up and running, much less build new ones. Others are being taken offline all the time because they are titanic Essence hogs and that energy could be better used on other things. So, getting the mountains of jade and other construction materials to the build site takes much longer than it would have in the First Age. Then designing the ship takes longer than it would have with Solars overseeing the project. Then building it takes much longer, especially because some of the automated factory-cathedrals are offline. What might have taken six months to roll out in the First Age takes ten years for the Shogunate to put together. This is assuming it's a priority project-it could easily take even longer. Remember, Dragon-Blooded crafting-speed multiplication Charms are awful.
Sidereals have decent speed-multipliers, but there are 100 Sidereals, many of them have zero Craft Charms, and how many priority projects like this do you think are going on across the entire world at once?
Shit! The Fair Folk forced a major breach in Mavais Prefecture! The entire city of Burnished Hope is lost— and they're turning around its wave motion cannon to fire it into the heart of the South! Deploy the Soulbreaker Orb!
Well, whew. That worked. We recaptured the city and broke the raksha army. Anybody know how to make a replacement orb?
What, we only have two left for this border sector? But that covers the territory of five different daimyos, who's going to have those remaining orbs stocked in their arsen… oh, they're going to war again.
Well, maybe the gods can help us get another one cobbled together. Dispatch a team of priest-diplomats.
Hey, they're back. What did Yu-Shan say?
They said what?
I'd have to summon a Neomah for that to eve n be anatomically possible.
In essence, the Shogunate would have probably stabilized at some point significantly above where we are now, but somewhat beneath the Autochthonians, if they'd managed to quit fighting themselves all the time. And if Heaven hadn't steadfastly refused to acknowledge the worldly authority of the Terrestrials. But "Heaven consistently tells the Shogunate to go fuck itself" and "Creation continually experiences cycles of widespread civil war and partial collapse of its bureaucratic and governmental infrastructure, often in the form of violent coups" are pretty much the defining features of the Shogunate.
So, the Shogunate would have been much more stable if it were not the Shogunate.
The Great Contagion did not come in and kick apart a healthy system that was stabilizing toward a sustainable state of equilibrium and renewal. The Shogunate was not something the Sidereals had under control. The Contagion just kicked the feet out from under a juggler who was already careening around, dropping balls like mad.