We've seen a lot of "civilization killers" and no one was able to get close to Civ6.
What I find funny, and this may be only my personal opinion, is that Civ6 isn't even that good and it's leading the 4X market by storm.
And you know, subjectively for certain players, it's a great entry (maybe the best), and it's a good landing zone for newcomers.
But I want to approach it objectively here - and I'm using the phrase objectively carefully.
I think most of us can imagine a Civ game that could be even better than Civ6.
It is entirely possible for a 4X game to be higher quality.
IF you had a flawless art style, and combined it with an appropriately strong AI (like Old World).
And it had less tedium in terms of the Religious system.
Perhaps a more intricate Government system. Maybe more impactful Spies. Better balance. More Leader animations and voice lines.
There's space for expansion here, which you might usually label as points for improvement.
Typically, competition for a game aims to 'steal' these points to become a better game in the eyes of the general audience.
But because competition for 4X is sparse, and making a 4X from scratch is expensive and risky, you won't find any worthy challengers for the throne.
This also avoids putting too much pressure on Civ titles to strictly improve between iteration.