Nothing matters about camera announcements until we hear hands on experience from folks in the field.
They'll tell you all you need to know about the camera and if it's worth buying or not. This will also allow for all the defects
to come out in the open. I personally wait at least six months before picking up a new camera body for this very reason.
In my experience, the higher density pixel packed sensors are great for things that have plenty of light to play with.
Not so much when low light becomes a variable.
Assuming you want / need a full frame sensor and using my own bodies for testing, I came to the following conclusions:
( I'm heavily invested in Nikon lenses, thus my bodies are also Nikon. Though I'm certain Canon will be similar )
All the math and theory is great but real world testing has shown me the reality of things.
The 810 and its 36Mp sensor is my go to body when you have lots of light, a need for large print sizes or the ability to crop in
on your target when you just can't get close enough. Keep it in the bag once it gets dark because anything beyond ISO 3200
is pushing it. Lightroom is good, but it has limits.
The D4s ( and its successor the D5 ) is my go to body when the lights go down or you need fast frame rates.
Pair it with a fast lens and you get the " Machine Gun that shoots in the dark " description.
A single photo at ISO 6400 between the two bodies and a noise comparison will tell you all you need to know in this regard.
The 810 and D4s share much of the same tech inside. So unless Nikon intentionally crippled the 810 for low light to prevent
competing with their flagship bodies ( a possibility ), the only thing that differs is the sensor pixel density.
In the full frame category, I am unaware of any offering by any manufacturer that is extremely high resolution and a low light
champion as well. I either have to pick which is more important to me, or buy two bodies to cover whatever needs may arise.