It this were true we'd have 1/10 of the critics we have now (which is what the number was twenty years ago).
There are more critics now because it's easier to be one because of the Internet. 20 years ago, you'd need to work for a publication to carry your story. Now all you need to do is start a free blog.
Critics absolutely have the power to sink a film. All throughout the week BvS opened people (not critics) were already anticipating a bad film and alot of them didn't even bother to watch it opening weekend (which is why all the estimates were off).
I think you have it the other way around. Opening weekend was packed. If people didn't bother to watch during opening weekend, then it wouldn't have opened to such high gross numbers. The decline is more telling. I think it's more on word of mouth rather than just critical reviews.