The key point here is youkai. Things will be better if we can keep them away.

Oh! Let's blow em monsters away to the milky way!

I got it now. We'll strike while the iron is still hot! Let's get on with developing the barrier sign without delay!

In other words, we need a barrier that "Conveniently blows away only youkai and welcomes only humans".

Since it's overrun with youkai, humans are scared to come here...

There is a truth in what you said, Sanae.

...Still.