If that was the way how it worked, then all apps would only have 1 star in total. Which is not the case since there's a lot of apps that have high ratings!
I don't see why this should be true. First of all, not all apps have bugs that seriously impact interaction with the computer. Second, not many apps have such a big user base. Also, note that the ratings are very polarised: its usually either 5 stars or 1 stars, with not much in between. For me, this is the sign that 1-star givers experience some really annoying issue that goes beyond simple 'like or dislike'.
Let's make a small extrapolation (of course, extremely subjective, but to give us a sense of the numbers we are dealing with here). Yosemite is running on over 30 million computers worldwide (and that is a very conservative estimate). Worldwide, there are around 60000 ratings given for it in the App store (and again, that is quite a conservative estimate, its likely to be less then that). Let's assume that the 1-star ratings outweigh the 5-star ones by 2:1 (not quite true, but definitely favouring the negative rewiews). Which means 40000 1-star reviews. Now, lets assume that 1 in 100 users (1%) that has experienced an issue will give it a 1-star rating. This brings us to around 4 million users with issues, or 13% of the entire install base. It is quite a high number and it clearly shows that Apple has some serious issues with their quality assurance and bug testing. However, its not a total disaster like some sound it to be.
And btw, its exactly contrary to what you say people on these forums are biased agains the latest updates. For one, because an existence of a forum will draw people that have negative experience, and in addition, because there is a bunch of very loud 'traditionalists' that share a strong dislike of the direction Apple is heading to.