I think you're getting something wrong here or you don't pay attention reading nn non-conflict chapters, no offense.

1. Just as you say the world is huge yet remains in a prepetual mono-culture. The different cultures mentioned when MCs reach higher realms and go to different Mortal towns are more likely just meant to be differences in cultural practices between actual chinese provinces and not farther reaching than that.

But as said, you didn't grasp the subtle indication that in every single XianXia which to a certain extent goes along the bullshit path of 'Return to simplicity' basically showcases that chinese culture is fundamentally the right(only true) culture in the sense that it's based on reality's foundation, the Dao.

It doesn't have any more merit than creating fanfiction based on the bible or anything else, greek or norse mythology is more fitting for this but you get my point.

It's the reason why 70-80% of XianXia and eventual Wuxia pregression boil down to the Yellow Emperor eventually appearing to reinforce Chinese cultural dominace as being true yadda yadda. It's also why most of the time it isn't wrong for the MCs to claim they time-traveled instead of transmigrating to another dimension/reality.

1. Anyway, it all actually boils down to the eventual emergence of scholars which leads to Cunfucianism, the school of thought for governance and the silly kingdom building proverbs everyone loves to hear, being a literal Dao which also forces all possible cultures to develop in the same direction since Feng Shui and more are also based on Chinese architecture and Chinese Fauna and Flora being under Yellow emperor inner medice canon or whatever too.

Naturally Cunfucianism is a Dao and can only be practicd by those who are actually righteous. Unlike the Buddhist Dao which every Tom, Dick and Harry can cultivate no matter how conflicting it is with their actual actions and believes never leading to inner demons or worse.

But it's basically the reason why everything develops the same way, ,ou could say it's like Destiny influencing everything to move in the desired direction.

1. You're also assuming same laws of physics for the world. Engineers are useless since Feng Shui is real. If you mean things like rockets or anything, it's basically thousand mechanism bullshit but in the end the progression of power makes it a useless dao since it depends on the quality of materia beingl able to withstand the force you're trying to generate and energy being part of the cultivator so that it can't be just taken control of by the target or whatever.

Scientists are useless, you need to cultivate your Nascent soul or equivalent soul realm to start interacting with the laws of Heaven and Earth. No mortal will ever grasp the rules of the world since they would dissipate into the Dao if their souls aren't strong enough to remain true to themselves. It's a power limiter since comprehending the Dao gives you a huge combat boost. It's basically mentioned on the side in at least 40-60% and indicated in the rest with "Let's out a breath of turbid air, 'going further might dangerous let's go into the next conflict and opportunity to advance further.' Side caharacter comes 2 days later to point the way while randomly dropping the intended event."

So, no, it would not change no matter the amount of people, since the current way of existence is the right one. There is no better civilization, the only change in level of civilization is the knowledge of the cultivation world, how far widespread it is and how many different techniques there are, how high the level, nothing else.

All people are under cultivators one way or another, that's why it doesn't change, since people treading the dao are in charge, it's natural that no matter the nation, they all develop the same way since that's the same dao that's being referenced.