Surely you can see that the Mistlands are devoid of content in a way markedly unlike the other main biomes

If you have not been there recently the place is now reportedly empty. This was done in the last patch to prepare for the Mistlands update.

Originally posted by GunsForBucks:

If you have not been there recently the place is now reportedly empty. This was done in the last patch to prepare for the Mistlands update.

Can absolutely confirm, Mistlands are totally vacant. No spawns, no plants, no nothing but rolling terrain.

Makes a nice quiet place for a house if you want to be left alone, actually. Might want to bring light sources, though. It's twilight at midday.

It was basically a placeholder biome, similarly to how the "Deep North" was nothing but icebergs, and the Ashlands was nothing but Surtlings. They were, however, intended to be used for later.

That said, the Mistlands is becoming the next biome installment to the game. The next major update will include it, hopefully in its entirety. I say that because, also, at some point they're adding cave-type dungeons to the Mountain biome. I'm unsure if it will be part of the Mistlands update, or a separate one.

So anyway, once Mistlands releases fully, Plains won't be the endgame zone any longer. ;)