The Chesterton area is in 3151 the only area the Capellans gained from the Suns compared to 2750. The Sirdar PDZ and much of the border planet's were Capellan in 2750. I agree with you that the map in 3025 is more important for comparing the borders between the Capellans and the Sun's, but he is correct.
I use the 2750 borders because that was the peacetime border and the time right before the start of the wars when all the states were considered roughly on par with each other, and ignoring all the worlds that dropped off the map in the course of the SW.
Although the Confederation temporarily gained New Syrtis, it has since lost it, and it has gained Chesterton, which while of great symbolic significance to the Confederation, is still ultimately just 1 world. There is a whole block of worlds up to Quittacas and Stein's Folly that were originally Capellan that are still in FedSuns' hands. In terms of material resources and value, those worlds add up to more than Chesterton. As a practical realist, I would prefer those worlds over the single world of Chesterton. As an aside, the 2750 Confederation border with the FWL has still also not been quite restored in 3151.
In 3025, the universe started with the Confederation already as the boxer on the ropes barely clinging to life (and getting further pounding in 4th SW), and its backstory is almost one of incessant loss. While the FedSuns had initial 1st SW setbacks, its story since then has been mostly of triumphs or at least steady advances with very little of its original territory ever being lost to anybody by 3025. If there were to be some form of reset, I would prefer one where all the states ended up more or less equal, and not one where one state is on life support as designated loser while another is the developer anointed winner. I don't see the Confederation or the Combine truly conquering the Suns, but I don't see the Suns being put back to an equal standing as a bad thing. If the SW were a foot race, the game started in 3025 with the Suns almost a lap ahead, and the Confederation running in reverse for a lap.
The Suns have spent the last decades in both game time and real time losing territory because they were built up to be this unstoppable military juggernaut that won whatever it put its mind to before so something had to be done to pare back the gains in order to maintain the setting. Perhaps STL doing most of the regaining in his lifetime was ham handed writer fiat, but the alternative might have been several centuries of incompetent rulers (like a Suns equivalent of Dainmar Liao?) and mistakes with the Confederation and Combine nibbling away at the Suns. That alternative could have led to supporters of both sides being dissatisfied. The craziness of Caleb Davion was another ham handed way of making the Suns stumble that I think could have been done better or more subtly. The trope of the insane ruler has been used too often IMO for all Houses as the explanation for less than successful rulers.
As for the Combine, I can easily see a civil war or shadow war between Toranaga and Yori, and this is what gives the Suns a breather or opportunity to regain territory. Succession crisis/struggle for the throne and digesting conquests for both Combine and Confederation is what I see saving the Suns from total disintegration, even if the Suns don't necessarily regain all lost worlds.
