The people that don't like [the developer's pick] are always going to be the most vocal. Using IRC as a benchmark of how many people they lost probably also isn't wise.
This is true, but the overwhelming apparent proportion still indicates that the actual proportion is likely fairly high.
The other two significant games of the genre* are doing quite well with nothing like Steam. Dwarf Fortress is hosted on the developer's site, Minecraft has its own servers. In fact, considering that fans of this genre are used to being able to get their games independently of Steam, this could hurt Terraria considerably.
I doubt it, in all honesty. DF and MC are both doing some higher level game design, IMO, especially DF. Terraria is just fun, and that's always sold on Steam at the right price. The appeal of this game as an impulse buy will crush the # of fans who know about it right now who aren't going to buy it because of Steam.
I don't know that you could make a significant argument for Minecraft there. The 3d building should theoretically increase longevity relative to terraria, but terraria has goal-oriented multiplayer potential that Minecraft doesn't. Other than that, I think there's no advantage Minecraft can claim that's not directly related to having been created earlier.
I'm not saying the Terraria devs couldn't have done it another way. They easily could have. It would have drastically slowed down release though. Choosing a distributor expedited the whole thing for them, and once the game got noticed in the span of a day, they put the emphasis on release and getting it out there, not on micro-managing their own release. _Plenty_ of indie games have gone the same route and done very well, there's no reason to think Terraria committed suicide by choosing Steam.
Certainly they can make a quick buck through Steam. But they've indicated that they'd like to continue developing, adding new content, and have Terraria be an enduring thing. I've never heard of a Steam-exclusive game doing that. Even those Valve games that had such communities have lost them since becoming retroactively Steam-exclusive.