This. The Imperium depends on war to keep its industrial base going and its populace occupied. Much like Oceania, it must fight to persist. If it cannot find heretics for the Guard and the Marines to die fighting, it will invent more as required.
Yeah but imagine all the things happening in between, and imagine the people who try making the first steps towards a post-war economic recovery, the kinds of people who stop melting down statues to make artillery shells, and start melting down artillery shells to make statues.
Space marines wandering, immortal, purposeless like ronin and errant knights.
Kriegers, happy, having lived to actually see forgiveness, trying to learn what it means to be human.
Catachanians become known as space australians, but otherwise very little else changes about their existence.
Ogryns become stars of Tallarn Rugby Roulette and household names of disreputable taverns.
The Adeptus Mechanics sends forth explorator fleets with renewed vigour, setting up new centers of documentation and archiving rediscovering all that humans lost on dead worlds, dead worlds now seeing colonization by a human population recovering from the endless war.
The Imperial Navy remains the same, as one of the very few institutions in the Imperium incapable of changing - but is bolstered by the addition of new capital ships, as the turtle's pace of Imperial ship production exceeds attrition.
The Inquisition, the Arbites, Deathwatch and the Sisters of Battle increasingly turn their eyes inwards to maintaining the political integrity of the Imperium. As mankind ceases to have common foes, the secret police ensure arguments do not become secession, and the scary face of the overt police in the Commissariat ensure any ambitious governors lodge complaints first with the Imperium (the complaints department is a bolt pistol).
The administratum attempts to sort out the Herculean effort of solving the Imperium's bloated bureaucracy and buried records, buried under millennia of more records, receipts and laws, but they actually manage to begin making progress - they set up a new department for managing the paperwork of paperwork managing. Nonetheless administratum morale is at an all time high, with request forms for reproduction pairing inspections through the roof.
Way in the upper echelons of society, governors, princes, admirals, administrators, dictators, captains, lords, inquisitors and fabricator generals begin the mad scramble for influence over their own private princedoms with which to call their domain, with an elaborate game of cosmic cloak and dagger from which few Imperials can survive for long. Business as usual more or less continues, but most planets find their Imperial tithe changing to knowledge, energy and humans - only now, the humans join the Imperial Guard to engage in elaborate civil engineering projects. Planetary defences are a high priority amongst such development, as old habits die hard, and men in fortifications die harder.
Hive worlds begin flooding with wealth and arts, like a neo-Shanghai in space now with more smooth jazz, as more and more of Hive society begins enjoying in the good life, free from starvation, xenos predation and culling missions, though the Ecclesiarchy, Mechanicus and Administratum still sends forth expeditions to "recruit" hivers when the staff lists are running a bit short. All the uncountable Imperial Guardsmen finally get to settle down on a world somewhere, some of them vowing to return to a home they once had, if it still exists, others just glad to get to die standing - this time, with a family. Leman Russes roll around farms with ploughs attached to the back, their tractor treads finding a place right at home on verdant agri-worlds.
Pilgrims travel throughout the Imperium, spreading with them ideas and fables, and no one dares scam them lest they fall into disrepute with the pilgrim's guards - great statues, that in a time long ago were called "Black Templars."
With the military necessity for Titan construction absent, a constitutional crisis occurs within the Adeptus Mechanicus as to whether the methods employed in constructing additional Titans is tech-heresy or a holy necessity.
The Ordo Malleus and the Ordo Ordinatus go around identifying, quarantining and clearing worlds that could become habitable, once all the unexploded doomsday weapons and screaming soul-foliage is cleared.
Imperial Death Cults and Khornate Death Cults begin a secret war that occasionally explodes into all-out murderfests, but no one else wants to step in and stop the disturbing murder goblins, as they otherwise stick to the shady undersides of urbanized Imperial worlds.
Remembrancers make a comeback, as the Imperium begins recording the time of restored tranquility, fully expecting war to return at any moment.