BattleMaster is never going back to the days when you could have a half-dozen people running a 15-region realm, and I can't see any rational argument for why it should.
Because 1) the main reason to go to war is expansion, 2) the player base is declining, and therefore 3) less and less realms are having a reason to go to war, other than to deal with TMP. Are you seriously arguing that wars are better now than they were before the estates system, that there are more and they are of higher quality? 'cause I would totally disagree.
And nobody's talking about 6 nobles maintaining 15 regions. But if a 12 region realm with 12 nobles makes less than a 6 region realm with the same amount of nobles, then you've got a serious design problem, and we may as well stick with the old system as the new one isn't offering any better. Realms with few nobles will still be penalized, whereas realms with many won't be advantaged anymore. That's the sole difference.
This does nothing to reward expansionist realms. This only helps realm with huge food deficits, which other than D'Hara on Dwilight (which doesn't have any rurals to expand to anymore anyways), are rather rare.
That's alright. As long as nobles think their income is likely too increase, expansion will occur. You can expect a reasonable rate of failure.....
Uhm, no. As long as the gain from expansion isn't obvious, many will refrain from taking the risks of fighting an enemy at the border. Just like right now.
you get more food though.
For 99% of the realms, food isn't an issue. Since food is sold dirt cheap on most continent, I can't see any realm saying "well, if we gain this new rural, we'll lose gold overall, plus all the war expenses, damages, and risks, but hey, we'll make 200 more bushels of food every harvest!". No.