You bought an Early Access game - a game that is playable at the time of it's release, knowing that it is still being developed. You did not pre-purchase a completed game. (Which means, yes, they are within their right to charge for the following biomes if they want to.)

The devs have said they are working at their own pace, which is their prerogative. It'll be finished when it's finished, basically. Keep up with the regular updates that they post to stay abreast of changes and new stuff.