I got to play Humankind Open Dev Beta.
I don't think any competitor usually outright "kills" another in the same genre. It's not like Street Fighter 5 killed any other game like DragonBall Fighterz or Killer Instinct, vis versa, etc. People tend to just buy them all and play one more than the other. When they get bored they boot up the other game for a little bit. The amount of hours spent playing could be your "killer" metric against Civ 6 or 7. Time will tell if that ends up being the case on Steam's metrics...
My take is that serious Civ players, especially those Civ4 purists that hate 1UPT, will probably play this game to death. The stacks are well designed and not even ridiculous. The reason why I say this is because the Humankind AI can be absolutely brutal hard. I lost several combat fights/skirmishes. I sent the Devs a feedback during my testing that they must make sure they have an AI setting that a casual player can handle. At one point the combat was so tough, I looked on Youtube and sure enough, Marbozir and Potato McWhiskey were struggling too. That's a really good thing in my opinion. Civ6 is too easy in a way, and too many guys complain about that. Humankind has the fights you are looking for if that is what you want. Humankind is a thinking man's game hands down. You have to pay alot of attention to the terrain, your flanks, your reserve units, your calvary units, and especially ranged units and the terrain they are on as well, AND the enemy's units placements on top of that! It's much more chess match than checkers. Winning a battle can feel quite rewarding, but might wear out some people if the difficulty is too high.
I think casuals will like this game alot too. It's quite pretty looking overall, the artwork is exceptional, everything looks new and interesting, the UI layout is nice to look at, the ambiance is relaxing, the music is very good, the movements are smooth, the events system is very well done and immersive (something I love from Civ5 Vox Populi), and the flow of the game is more like your own story much more so than how a Civ game makes you feel. I think that story element will really lure in the casuals more than anything else. Civ feels much faster than Humankind and some people will want that faster turn play, but many people on this forum at least, love Marathon and Epic game speed, so....maybe that won't be an issue.
At the end of the day, competition in the marketplace is good for us the consumer. We win in the end. I hope Firaxis steps their game up on Civ7 because if they make another game just like Civ6... it won't be the same acclaim and probably not the same in terms of revenue at release. I can only speak for myself, but I was not going to automatically pre-order Civ7 going back last year. Now that I have played Humankind, I will NOT be buying Civ7 at all unless they give me a good reason to buy it in terms of mechanics and AI which I care the most about.