The metaphor of what a "Dragon" is is a natural disaster.

In 3.5 (maybe dragon magazine) I remember reading that when dragons age out, they don't "die" they become landmasses like hills, forests, mountains, rivers, etc. I think it also said something about them needing to collect wealth for this reason (not sure on that)

I bring up the dragon, because dragons are literal natural disasters in D&D.
Godzilla was a metaphor for nukes destroying japan.
So these beasts of great power, which you've labeled kaiju, are similar.

Then we come to the PC's. The characters who advance to the end of their level career are supposed to be so strong they can stop natural disaster or be them, so yes, PC classes are supposed to be a man capable of becoming a kaiju (or at least thier power level)

I'm not sure if I answered all your questions.