The Solomani Pretty much rolled into the Old Expanses, because well, the Imperial fleet left. That did not mean however that the Imperial worlds they encountered necessarily greeted them with open arms. Taking ground is one thing, holding ground and making the people "love" you is another, as we saw in the Iraq War. The military occupation of so many worlds in the Old Expanses, the Solomani Rim, and the other portions of the Imperium they seized would have been a strain. Add to that the major fleet engagements they would have had in the Solomani Rim, Daibei, and Diaspora, and there would have been a further draining of resources. Remember too that within the Solomani Confederation while most all the factions shared an obsession with retaking Terra, once that happened, that source of unity was pretty much gone, indeed among the moderate Solomani factions (e.g. the Booteans), there wasn't a lot of enthusiasm left for the war once the easy victories dried up. Hardliners on the other hand wanted to reconquer the entirety of the Solomani Sphere and a few even thought it might be possible to conquer the Imperium itself. Squabbles broke out over war aims, moderate elements started recalling forces, hardliners starting working toward purging the "disloyal, defeatist elements", that lead to even more resentment and fraction within the Confederation, and civil war or at least a revolution was on the horizon in 1130, Virus or no Virus.
But even if the Solomani had been unified in their war aims and not been on the verge of a major split though, exactly how does the war end for them? Daibei wasn't going to surrender. Lucan's remaining forces in the Solomani Rim weren't ever going to surrender and the Vegans didn't figure on surrendering either unless it meant their annihilation, since they knew what would happen if Solomani hardliners ever got to be in control of their worlds (punished for "collaboration" anyone?). Then there was the whole morass in the Old Expanses where Margaret's forces caused trouble and showed no signs of being willing to stop. Trying to push deeper into the Imperium would be a fool's errand. At some point the Solomani would have had to declare a unilateral cease-fire and set up some sort of defensible border, and that would have been extremely challenging.
But even if the Solomani had been unified in their war aims and not been on the verge of a major split though, exactly how does the war end for them? Daibei wasn't going to surrender. Lucan's remaining forces in the Solomani Rim weren't ever going to surrender and the Vegans didn't figure on surrendering either unless it meant their annihilation, since they knew what would happen if Solomani hardliners ever got to be in control of their worlds (punished for "collaboration" anyone?). Then there was the whole morass in the Old Expanses where Margaret's forces caused trouble and showed no signs of being willing to stop. Trying to push deeper into the Imperium would be a fool's errand. At some point the Solomani would have had to declare a unilateral cease-fire and set up some sort of defensible border, and that would have been extremely challenging.