It's not like Spirits didn't already knew how to enter the human before.

They have a point.

We've seen already that most spirits seem to not like, or outright hate humans. Opening the two worlds will cause problems. Like Wan Shi Tong skinning anyone who goes near his library. The spider-spirit dragging people away. Or Koh, stealing faces all day and night.

That's not even mentioning what the humans will do. Wan's story showed what humans would do when given an inch, and it's not like the ones of Korra's time have had much reason to be all happy-peacetime with them.

So yeah, things will likely go very bad before they actually get better. And it'd be really damned stupid if Book 3 starts with everything magically being sunshine and rainbows for both sides.