That is completely not at all a reasonable assumption. There is no evidence to show that people complain, then buy the game. Looking at the basic figures doesn't even tell you if the same people complain are also the ones who buy.
Do we have evidence of this?
Neither do I. The worth of the game is irrelevant to my point. My point is a lot of people seem to be complaining bitterly about issues and then buying the game anyway.
As I said before, the evidence is largely anecdotal. But given the vocal complaints that are kinda hard to miss on the web coupled with the huge sales figures, I think it's a reasonable assumption.
I think a fairly large number of those complaining are complaining because they bought the game (Most likely with the belief that Blizzard, a company with vast experience running online games, would be able to pull always online DRM off without too much hassle for the consumer), its been pretty much unplayable for 2 weeks straight in europe (atleast on evenings and weekends when people are free from work/school), From what i understand the situation with the asian servers aren't much better, the US ones seems to be "ok" but also had some major issues closer to launch, Throw in the "controversies" with the RMAH (reducing droprates on items that alot of people are trying to sell to force people into buying gear rather than farming for it) and it becomes quite obvious why alot of customers are upset.