tl;dr: My opinion: "Stretch goals can grow your Kickstarter's income linearly but you grow your project's complexity exponentially."
So far Kickstarters have had good success. We are still waiting with anxiety for the first big failure.
Obsidian and the leaders of this project really have built up a lot of credibility and have a great track record. No doubt Project Eternity will be great.
But there is something that worries me when I see more and more stretch goals added to a Kickstarter. As a kickstarter you pitch the project with a certain focus. You don't know how popular it's going to be aka whether you'll have money to build it. So, like Obsidian you pitch the general framework, promise some features and go with it.
In PE's case.... HUGE SUCCESS. Amazing flow of backers and money. Awesome!
So... now we need stretch goals. Yes. That's the way Kickstarters work. Need more inspiration to keep getting more money and make the project what everyone wants it to be.
But wait... Do you throw in crafting? Multiplayer? 3d steroscopic drivers? On-line PVP? End-game max level reward system?
I hope that as stretch-goals get added a company like Obsidian is very cautious about what will be promised as a feature. The more features, the more complexity. The more time it takes to get everything right. My best wishes to Obsidian to take care in this.
I really want this to be a fabulous game. There is a hole in my little gamer heart reserved for another game to come along and please it the way Planescape Torment did!