You're both assuming that realms would be distributed evenly, but there would be clusters, especially when a realms split up. Moreover, in my year on Dwilight we never once went to war with any of the realms that we shared a border with. Instead we were in a perennial state of war with Astrum, despite being separated by about 10 rogue badlands, and Corsanctum, despite being on the far side of the continent with an ocean in between.
Not that your point doesn't have merit, I agree that my idea could change the way wars happen, but if some realms were so insulated from others, I think you would see more intra-realm conflicts leading to successions, and you would see ambitiously bellicose realms shifting to get closer to their enemies. Capitals might be located near the frontier rather than in the centre. And if that still wasn't enough, you could allow out-of-capital recruitment, and the option to refit in allied realms. You might see more infils and traders too. And my idea has a built-in incentive for players to recruit their friends, because they need more people if they want to conquer more territories. Finally, it puts the players in control of their destiny.
