I don't understand the words of Reimu that if Oni waited something something and you were too hasty.

If I'm reading it right, it means that humans grow in power as the generations go on because they have the traditions built over time as a base. So if the Oni hadn't left to go underground, the humans eventually would have improved enough to face the Oni directly and give them challenging fights (which the Oni love).

Instead they vanished until the present day when Suika came back and the humans changed in different ways since they didn't have the Oni to go up against.

That's my take anyhow.

If I'm reading it right, it means that humans grow in power as the generations go on because they have the traditions built over time as a base. So if the Oni hadn't left to go underground, the humans eventually would have improved enough to face the Oni directly and give them challenging fights (which the Oni love).

Instead they vanished until the present day when Suika came back and the humans changed in different ways since they didn't have the Oni to go up against.

That's my take anyhow.

Correct. As with the author said in the afterwords, (paragraph 5 & 7) human's power is the power of culture, civilization, tradition, and concentration.

Suika does it all- freeloading, romantic advice, immortal counsel, alcohol reviewing and trash pickup. For one of Japan's most feared demons, she's pretty nifty.