The societal dependence on responsibility, including caring for children, would be weak.
I'm willing to wager that most intps think of child rearing as fundamental. I'd think that many intps have spent a lot of time thinking about the complexities of raising a child...and the "best" way to go about it. It is, after all, an interesting issue to think about...and certainly central to the well being of humanity. (And most intps are driven by that...inferior Fe and whatnot..)
it would most likely resemble something like world wide hippie community with loads of small villages. weird artwork on the side of the roads. probably not too high tech, because we wouldnt have the factories etc filled up with other types and this would hinder the development of technology quite a lot. people would generally not mind too much about other peoples business(like if someone lying naked on the side of the road sunbathing wouldnt get asked "what the heck are you doing there?"), but would help the stranger or who ever needed help(we are Fe creatures after all and the younger ones would learn the value of helping people in need from the elders, who already developed their Fe). we would be working together as the village, some people would grow the vegetables, others would breed horses, others raise cows and milk them, others inventing weird gadgets and build them for the people along with their "pupils" who would work as the labor and also be apprentices, later teaching the younger generation etc. there wouldnt be governments or kings, instead there would be town meetings and people would discuss whats the most rational thing to do about some subject. people would respect the wisdom that the elders have and listen to their opinions, but the elders would also listen to the viewpoints of the younger people. also the people from the next village would be considered as equals to people in your village and there would be exchange of ideas between villages(like if someone in your town invents some useful gadget, he would go tell about it to other people). some people who already learned a lot about some subject would travel around the world and spreading his wisdom around. there wouldnt be overpopulation, not because people wouldnt talk to each other(i know you started thinking about that ), but because there wouldnt be resources for small village to double its population each year, so it wouldnt be rational to have so many children, also people wouldnt feel so much urge to get kids, because, well they are quite annoying to many INTPs , kids would be made to keep the village alive, not for entertainment "mr.zingabunga died, who wants to make kids?". people would teach their kids the basic maths, how to read and write etc, so there wouldnt be much of a need for schools, instead kids would go to the people who do things they are interested(if they accept you there, not every kid in town can be at the chicken farm all day) and learn from them, help them at the chicken ranch or what ever when needed and later start working full time there and possible get to be the chief chicken farmer some day teaching kids who are interested about it and guiding the workers. naturally the world wouldnt be too busy place, people would have at least two hour siesta every day. people working on the chicken farm would get chicken as much as they and their family(kids, parent or disabled brothers and sisters etc) need, but there would be enough chicken to give for other folks in the village, in return they would get corn from the farmers etc. food distribution would work quite systematically and everyone would have something to eat. there might possibly be some restaurant type of place to make things easier, so that everyone(or no one except the chefs) wouldnt have to cook food. naturally the houses would be built by carpenters, who would also fix things around the village and make the weird art besides the roadside, if there wouldnt be bridges to built or houses to fix.
"Where wisdom reigns, there is no conflict between thinking and feeling."
— C.G. Jung
We would all starve to death because nobody would want to do the Sensorish work of growing/collecting/raising food.
We would become massively inbred or go extinct because so many INTPs would be unable to have a relationship with another INTP (or anyone).
We would learn all sorts of things about how the world works but our life would never improve because it would never be put into practice, making the knowledge useless.
And the few products that might end up getting produced would be horribly unreliable to the point of being dangerous, because no INTP would want to do the tedious task of quality control.
Not to mention there would be nothing produced which couldn't be manufactured by robots, as INTPs would be uninterested in factory work.
And we'd probably die young because so few INTPs would be interested in a Feeling-oriented job like a doctor or nurse, not to mention the tedious work of developing vaccines/medicines.
On the plus side, there would probably be no war, because if two people disagreed, they would just have vicious flame wars online (at least until they got bored of it). Of course, given the work involved in manufacturing computers and keeping the internet running, it's highly unlikely that we'd have an internet in the first place.
EDIT: although, there is enough variation within INTPs that none of this would probably happen, as people stronger (weaker?) in one function would take over those jobs. Still, as it wouldn't be their strength and as the number of interested people would be small, the world would end up suffering.
I disagree, I don't think the world would be that dire. There is still differentiation even between people of the same type. Some INTPs will be stronger at the sensing and feeling skills than other INTPS and they would be the ones likely to put the ideas into practice and serve as the doctors and nurses of society.
5w6 or 9w1 sp/so/sx, I think
@SuchIrony, that was mostly a joke/mocking the OP. If you were around back then and knew the OP, it'd make more sense in that context. In reality yes, I'd tend to agree with you, although I do think it would be harmful to society overall (as it would be for any other type).