You also fail to consider those down stream. By absorbing the water in Missouri, the flow rate is dramatically slowed. The volume of water arriving in Memphis and New Orleans will be the same. The rate at which it arrives and the flood levels will both be diminished.
The decision is much broader than some farms in Missouri or a ghost town in Illinois.
The farmers bought land in the flood plain and should expect floods. The levees do not eliminate the risk.
The blasts did nothing to ease risks downstream, where the Army Corps of Engineers warns that the Mississippi River could rise to its highest levels since the 1920s.