True, we couldn't do it those days, but not because we are more stupid. There is no way we can get this many people together to work on a project like that. Neither would there be anyone wanting to invest that kind of money in a project like that.
But those are all reasons of changing attitude and society, has nothing to do with the degrading of humanity.
It is easy to overlook the "miracles" of these days. The industrial revolution, miniaturisation, computers, we landed on the moon even! We can land (well almost) robots on Mars. We can make robots.
Nothing of that has to do with the number of days in a year or with pi.
I think the mystery of the pyramids isn't how they knew those numbers in the first place, but why did they put them in a pyramid? What use is that?
Mysteries aren't called mysteries without a reason. We don't know enough about it. And assuming that the Egyptians were way smarter then we were is only one out of many possible answers.
There is no way you can proof that mankind was smarter the further back you go.
There have always been geniuses and dumb peoples and there always will be.