I was actually a buyer of the Early Access Alpha and I am incredibly happy they put DOS on Steam. Why? I missed the Kickstarter. Am I mad that I paid 40 dollars for an Alpha? Absolutely not. The game in its current state is fantastic even with the bugs. My only regret is I can't play as much as I want because it is isn't finished. But I'm excited as hell to see what comes next.
Unfortunately the Steam Community boards are a cesspool of trolls, whiners, etc etc., and the whiniest, trolliest of them all are the loudest. Don't get me wrong, there are good posts. But as mentioned by OP, the majority of these posts are negative. However if you look at all of the reviews on the Store page, the overwhelming majority of them are positive and the game comes recommended.
Early Access is a double edged sword. It generates more hype for the game, gets more people in on testing, and generates additional revenue that I assume helps support the developers. It also gets unfinished products into the hands of more people who don't understand what they are buying into thus generating more hate. I have however seen this same type of hate generated from other Kickstarters as well prior to them going up on Early Access. In my opinion, it is less a problem of the delivery vehicle and more of an issue with the community as a whole. I can only hope that despite the complaints, DOS sees well-deserved success.
