No one said it was a massive failure, but it is clear you don't know how to read box office numbers.
It has actually become very common place the last few years for these films to stack. That is a problem, but the films that get great WOM have legs. They aren't the same kind of legs films use to have, but this is bad.
Even if Godzilla had opened with a 60% drop, it would have been bad. It opened under $100m, which means it is not falling from the mountain top. A movie like Catching Fire or IM3 can fall 70% because they open so big. Godzilla's opening wasn't close to that.
According to who, you? Because it is being reported as a huge fall for a reason. As Mojo put it,
http://boxofficemojo.com/news/?id=3848&p=.htm