This is the impact of war.
From the outbreak of the Iron Armor War to the conclusion of the second battle of Reiaductus, this continuous 15-year war devastated the Spiderweb Domain. The Thistle and Mustard, Alfonso, and Aloft Claw Star Domains were relatively less affected, but their populations still plummeted sharply.
The expected 17 trillion theoretical tax currency is certainly gone; having 1 trillion would be fortunate.
With the population base decreased, the Imperial Tax will naturally decrease as well.
Moreover, although the Iron Armor has been eliminated and the Alfonso Sect disbanded beyond recovery, leaving no one to intercept and collect taxes.
