All of those factors fuel sales. And Steam reviews have a secondary function, which is that they boost developer morale.
Vault Dweller is of course a man of steel (iron?) who cannot be touched by positive or negative reviews, though perhaps even he is affected by them to some extent, and I suspect his teammates care about them.
In the absence of any other metric, Steam reviews are a good indicator of players' engagement and support. You see a game with 70 reviews, even if the rating is 90%, the game either didn't appeal to many players or flew under the radar. You see a game with 30,000 reviews, you know it's a hit even if the rating is 78%. Our games will never get that high, of course; so what's left is the rating itself. The game quickly dropped to 84%, then slowly climbed up to 87, then down to 84 over the last 2 months. Two negative reviews are due to the lack of key binding, which is coming next week; whether or not the reviewers would adjust the reviews remains to be seen. The vast majority of the negative reviews are due to combat difficulty, which isn't going to change.
I'm not complaining that some people don't like the difficulty; that's both understandable and expected. The question is whether or not we we get enough support from people who liked the difficulty and the overall design. AoD kept 81% rating no matter what but the graphics kept a lot of people away. Colony Ship has a much wider appeal and thus can ended up much lower, which would determine everything else, from the tentative sequel to post-release polish. No reason to make a sequel to a game with a <70% rating or to spend 6 months after release on minor improvements. It just wouldn't be feasible due to 5-6 years development schedule and nearly assured similarly low rating and thus low sales.
I don't think we'll stay at mid-80s for long but it would be nice not to slide into the abyss below and in this battle every vote counts.