As usual the answers are highly subjective.
I can answer only for myself:
1. Are 18 Megapixels enough?
YES! I prefer to see them improve DR and low noise
2. Why do we need more than 12 stops of DR?
So as to try to come as close to our eyes (Perfect for landscapes.)
3. What ISO setting will we need to take pictures on a new moon night? Do we really need anything better than the 1D X / 6D?
I do for shooting landscape astrophotography. Can I have 1M ISO so as to have a super clean (10-100K)?
4. When will mirrorless camera take over? Bonus: When will they pass the Turing test???
Probably never, or, in 4 ,5 decades. Who knows, who cares
5. Assuming Sony, Nikon, or Fuji had the same lens selection as Canon, and money was no issue, would you switch? Why?
No I would not
6. Is IS necessary on wide angle lenses? Do you even use IS on your lenses?
It is a welcome addition. I use it when I have it.
7. What's better, USM or STM?
I do not care about video so USM I guess
8. Should DSLRs have video or is just a waste of R&D & money?
They can have but it should not be the predominant feature.
9. Is DxOMark THE authority on lens and sensor measurements?