Well another thing to consider...
Firaxis still has at least four months to release uncontested content. So that's just free money before Humankind even comes out.
And then, on top of that, Humankind kind of has a pretty basic starting roster. I think the only things it offers as far as new cultures go are:
* A few ancient civs that Civ would never touch like the Olmecs and Harappans
* Some "split up" civs like different eras for England, France, Germany, China, Persia, and India. Which again civ doesn't need to touch and mostly has with alternate leaders.
* Weird alternative civ choices to fill out eras that not many people wanted like Mexico (which we already have Aztecs and GC as better substitutes) and Ghana (which we have Mali with more personality).
My point is, that Civ already covered all these basics, and it is highly unlikely to be looking at Olmecs, the Mughals, or Mexico for a second season pass. Which means, while Humankind will be released and just barely getting to baseline, Civ VI can still have unique things to offer that Humankind doesn't have.
Players simply won't have Bulgaria, Berbers, Burma, Navajo/Cherokee in Humankind. And likely won't ever or for a long time. So whatever Civ would likely release next year, outside of Assyria and Austria it would all be content unique to Civ VI that Humankind still can't compete with. Humankind will still likely do okay with players who want more streamlined mechanics, but Civ will still be the game with all the new shinies.