You do know that macro is mostly all manual
No kidding?
I would have never known that but for this post

and that the MP-E 65 can be used with adapter on other cameras?
Realistically, the MP-E 65 mm is the single best, most versatile lens for macro shooting, especially out in the field.
I have never heard of it used on other cameras, and especially not on a Nikon. Please show me examples.
I know for sure it is usable on the end of a Canon, without any adapter

From what I've seen specialized macro shooters mix and match optics and accessories from a wide range of brands all the time.
This is true, especially in photomicroscopy.
People use diopters, Raynox, tubes, etc. ... but (in every other system) to switch from 1x to 5x means
changing tubes, lenses, etc. ...
unless you're using Canon and have the MP-E 65mm, where you don't have to change a thing.
While ALL systems can be set up for indoor studio stacks, including Canon, no other system has the 1-5x
field flexibility of the Canon MP-E 65mm on top of this.
For this reason, the lion's share of macro shooters shoot Canon. And, until somebody else comes up with an equivalent, I suspect it will remain that way.
Hey, I too had to stop and think about the D810 sensor, like everyone else, but ultimately the question was,
"Why would I want to complicate my macro life, and minimize my options, by switching brands?"As can be seen in the photos above, the image quality in the old 40D, 7D, and 5D is plenty good to produce colorful, clear, mind-boggling macro imagery ... and the 7D II will be better than all of these ... while still giving me all the flexibility in the world, with the MP-E 65. Why would I go through the hassle of switching to any other brand, just to make my choices more limited and difficult?
I will be able to have my cake and eat it too, and I like the sound of that

Jack