Yes, Japan has a withering population. However, do not underestimate their wealth, technology, and manufacturing capability.
The mathematics of war are changing rapidly and those three things can make any nation who possesses them a formidable foe.
If Japan were smart they would develop numerous squadrons of stealthy, pilotless drones that would make a sea based invasionof Japan impossible. Stealthy, hard hitting drones that can be launched from cruise missiles,planes and submarines as well as land bases have made suface combatants and armor formations obsolete. Technology has altered combat realities beyond the perception of most senior officers in the world’s armed forces. Many young,brave men will die horrible deaths before the new realities are understood and appreciated.