I like the manga actually going out of its way to show that the shounen logic that Shin has is backed up by actual thought instead of Naruto logic and that he's completely aware of its deficiencies rather than just pretending the deficiencies don't exist and that disagreeing is a moral failing. I also like that the manga makes the people disagreeing have a balanced opinion and actual logic as to why they disagree with the softness.
The magistrate's speaking from five hundred years of hard-won experience on what does and doesn't work. More, even, since Zhou China wasn't exactly a utopian land of peace and plenty. There were probably generals just like Shin in that time, but they all died, because it's a flawed idea that doesn't work well. So the fact that there's real difficulties in the "other path" lends weight to the idea that it's not that Ei Sei, Shin and Tou are uniquely good individuals in the history of China but rather that the "evil" path is the one that's actually worked out for the generals in a position to choose, so that even otherwise good or neutral generals have had to choose the way that's not sugar and spice. Because of course, even most people reading this manga would probably choose Bihei over Onkei if you had to choose one of them to die in a gutter somewhere.
I especially like how Tou immediately asks the magistrates, who are experienced with controlling people and the administration of cities, for help. It makes the choice a meaningful and deliberate choice to take a gamble, not on the moral fibre of your leaders or your men, but on the administrative ability of your subordinates, rather than retroactively painting everyone who came before as bloodthirsty imbeciles. It's not that nobody else wanted to do what Ei Sei's attempting right now, it's that some of them tried very hard and failed. I think that's a lot more compelling, because it shows respect to all the people who came before.