As someone that usually plays single player with like 18 factions on huge maps, I don't find that having more factions makes the game itself more complex, it just means you might go to war earlier really.
If we take the diplomacy of 7 and say you start on a map of 18 factions instead of 5, sure 18 is more complex. But if we are comparing 7s diplomacy but with 5 factions vs civ 6 with 18, I think 7 is more complex, because you have many more options for influencing the other factions. Like, 5 swiss army knives to me is more complex than 18 just regular knives.
Also I've always found terrain to be pretty "samey", like each continent had unique resources yeah cool that was basically it. Maybe we will have bigger bonuses for off-continent resources or something like that, giving you more incentive again to interact with others.
Another reason for playing 18 is that I find it more immersive. However immersion and complexity are different topics.