20% of participants who say they won't buy the game because of Steam sounds like quite a lot.
The number it´s a bit higher than 20 % if you want to extract the information how steam will affect the sell figures. You have 20% already now declaring they wont buy the game, you have to add 4.55 % for the persons "probably NOT buy the game, Steam is making me less likely to buy it". So the guess only using this data would be already a fourth.
To use a bit more of the informations, you could also divide the undecided but objections toward steam persons using the partiton atm observed between the steam has a negative affect and will probably buy / will probably not buy factions. 17 of the "Steam is making me less likely to buy it" are undecided, 14 (edit: now 15) will probably buy it and 7 probably not. So lets guess (in sense of data driven estimate) 5 of the 17 undecided will join the probably not group.
Now we have a guess of (5 + 7 + 32 =) 44 persons which will probably not or not buy the game, and steam is at e last making is less likely to them or is the reason for the decision. So the guees would be a bit over 28 % (44/154 = 0.2857)
But this is only one side. 14 "will probably buy the game, Steam is making me more likely to buy it" and 5 "will definitely buy the game, because of Steam" form the opposite faction. The don´t really care faction, alias "Steam doesn't affect this decision" and will buy the game, but "Steam is making me less likely to buy it" aren´t member of neither of the sides because they would buy the game also without steam. So we have 19 of 154 where steam is the reason or has at least a postive effect, or 12,33 %.
Naturally 32 out of ~150 isn't all that many, so while I sympathise with them, it seems to be not more than a vocal minority and not representative at all of the general public.
A sample size of 150 can be quite good, tbh some studys would be lucky if they had such a sample size. The more important question is how representative the users here are for the entire customer base of civ5, and if the the potential difference will affect the answer behaviour toward this question.
One point is, person who don´t care about game communities, only play unmoded single players games, will be most likely (this time it´s a simple guess, nothing really educated or data driven) a bit underrepresented here. Another point could be that persons with objections toward steam are a bit more active atm. So there could be a bias to both sides.
But finally to your point "not more than a vocal minority". Following strictly the data, the real vocal minority are the person who want steam included, followed (but having already doubled in numbers) by the person with objections toward the implemention of steam and the biggest group are the "steam doesn´t affect my decisions" or "will probably buy the game, Steam is making me less likely to buy it".
Edit: who voted while i wrote this post
