I was part of the paid alpha for Everquest Landmark (another voxel-based building game) and they had that exact problem. The people who were paying for access had no intention of actively helping to find bugs or providing feedback, and didn't appreciate the game was in an alpha state without most of the features. They just wanted to 'play the game early' and then whined about bugs, bashed the game on the forums, it basically (in my opinion) turned into a huge mess in the end which is a shame as the game showed some real promise (when they fixed the ability to actually claim any land!).
I don't know how much that experience contributed to the collapse of the game, or if the developers just had their funding pulled for other reasons, but it is a hard line to walk when you need lots of testers due to the scale of the game, yet you want genuine testers with patience and not angry kids demanding fixes and bashing the alpha because of a lack of features.
If the KS goes ahead I will be one of the first to sign up though and I understand what you're saying about the mystery, but to me it's all about permanence, so the magic of the game is still there because on day 1 of the final release the universe will have been wiped so only at that point will what I create and do have any permanence, making it all exciting again