*Stretch fingers* You know that the cotton situation is exactly the one most of the Western World is in right now? If trade stopped, countries like France couldn't survive a year with its agricultural production because our economy is based on an interdependence with the rest of the world.
Also it's easy to say plant food crops, but where do you buy the seeds? If everyone switches at the same time then you will have a shortage and if not the prices will skyrocket way beyond what regular farmers can afford. The idea of Souma is not "You can't eat cotton so plant food duh", it's "the government (which until then was indebted beyond reason) will provide (trans. Buy) the seeds not only of regular crops, but also of fast growing crops that were not available until then." It is a level of state intervention that is unheard of in a feudal society, so yeah they can be surprised. Feudal lords aren't in charge of what the people grow, they only rent the land and the peasant do what they want as long as they pay the rent of the land (and with the cotton industry, that means that it must already be on a coin base instead of crop yield, hence why they didn't see the cotton problem).
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MangaA-Z Same thing, in feudal society people live by renting the land and here Souma propose that people live free of charge in what is basically is a colony. Which means that he has to pay (again debt problem anyone?) the buildings, the infrastructures, the security, sanitation etc... and go way beyond what a regular feudal lord, an even more a king, has the power and money to do. And it's not a case of return of investment, if it does it will be in decades.
And finally the preemptive strike is impossible in a Feudal world since there is a simple thing called Casus Belli. Declare war without one and all your neighbors will rip you to shreds because you don't respect the rules, and that's especially true in a world where you have a USA-like superpower that is the garant of this order.
And also if you read this chapter you would know that Souma has basically no air force since Castor rebelled so good luck with that...