This is why balance is unnecessary in an open-ended game.
I don't think game fairness (which is really what we are talking about when we talk about balance) depends upon being able to defeat foes. A threat could be totally impervious to PCs and still useful to them. Consider the hypothetical invincible monster (such as, for example, the Dungeon World version of the Tarrasque). All you need to do is figure out how to get the paths of your other enemies to cross with the Tarrasque, and it will do your dirty work for you.
The same is true, of course, for even the most devious traps or the most deadly hazards.