The point is that only being able to recruit in the capital is a gameplay balance decision, and as such, circumventing it by moving your capital (or seceding) specifically for that reason has no IC justification. It is purely an attempt to circumvent a deliberate balance decision.
If we change the decision on balance, then there's no longer a reason to restrict secessions and capital moves, because there's no longer a recruitment benefit to doing them.
Well, you mentioned that having the recruitment fair anywhere else than the capital would be very costly in upkeep, so the incentive to move the capital to the front would remain the same.
Strategic secessions and strategic capital moves are both risky for their own reasons.
I consider the secession of Iato to form IVF to have greatly precipitated Enweil's death, despite being a friendly secession. In theory, both realms had higher tax tolerance, had two capitals to recruit from, and thus were better off. But Enweil was at that point as almost all realms are now: devoid of enough both capable and active players to run two realms efficiently, if it even has enough for one.
Let's just think of generals and marshals here, since it's the most blatant example. How many realms have enough competent people to run multiple armies efficiently? Barely any. Heck, I'm sure we can all think of a ton of realms that can't even run a single army efficiently. So what happens if you split them up? They'd crumble. Two poorly run splinter realms would have no clue as to what the other is doing or enduring, would not react in a timely fashion to any invasion, would not be ableto coordinate joint attacks, would have much longer refit cycles, etc.
In almost all cases, strategic secessions are an unstrategic option.
But what about the few that would be strategic... so what? Wars have become too rare for many parts of the game world. Are colony takeovers even a thing anymore? Do they still exist? What's the difference between a normal takeover of an enemy city and then seceding it to form a new realm at the front, and a colony takeover? They both allow to continue a war where distance might have otherwise rendered it impossible to continue, or would have caused extremely long and boring refit cycles.
