utopian societies in the modern world are nearly impossible due to the varying levels of development, diversity in culture, and our complex world economy.
For the sake of this hypothesis, let's say that our Utopian Earth is a place where each country is equal, no person is undernourished or discriminated against, there is no war, and the labels of "rich" and "poor" have been demolished.
It seems a bit much to ask for.
Due to the location of resources on Earth's surface, as well as the different early civilizations of man, the world has many different settlements, all varying in their development. There are both more-developed countries (MDCs) and less-developed countries (LDCs), both of which providing for separate needs for the people on Earth. If there was a central government over the world that met the requirements that would dictate it a Utopian society, these needs and means of providing for them would be altered dramatically.
Currently, the location of most physical labor, such as agriculture and manufacturing, is in LDCs. These countries also have the highest crude birth rates and crude death rates in the world, due to lack of good medical care, nutrition, and education. Most LDCs are located in tropical areas, where mosquitoes and such do not die in the winter, meaning that the risk for malaria and other diseases carried by insects usually remain the same year-round. Their location also affects their available resources, which are relatively little.
On the opposing side, MDCs mainly have jobs focusing on the consumer, and have easy access to good education, nutrition, and medical care. They are usually located in temperate climates, decreasing their risk for certain diseases. Sadly there is not much to say about MDCs except for that they're usually much safer and cleaner than LDCs and do not have a need to hire people within their country to perform physical labor. They typically get this labor cheaper somewhere else, like LDCs.
Now that the lengthy explaining is aside, let's talk about how this affects the idea of a Utopian Earth. The idea is quite simple, but the reasoning behind it can get a little bit tricky: Less-developed countries would need to develop at an unusually high rate in order to be treated equally to more-developed countries, and would need to do so without collapsing.
It's not like LDCs have no chance of developing, it's just that it takes time. America developed in around 200 years, which is relatively quick in the scheme of things. In order for a Utopian Earth to become a reality in the near future, LDCs would have to reach our level of development in a short amount of time, which would be highly problematic. The government would most likely promote urbanization, or increasing in the number of people who live in urban settlements, which would lead to overpopulation in said urban settlements. There would not be enough jobs in these settlements to compensate for the people.
This all would need to happen without wars between tribes and nationalities, as well, but that's in the next section.
Let's also note that LDCs are usually much poorer than MDCs and cannot afford good medical care on more than an individual level, and would need to somehow decrease their crude death rates, and most likely enforce laws to lower their crude birth rate, all while their people are suffering from horrible diseases and spreading them due to lack of sanitation.
Like I said, it takes a lot of time to develop, so a Utopian Earth would not happen for a long time if we let LDCs develop at a "normal" rate. We don't want them collapsing and taking with them their cheap labor and exported goods.
Gosh forbid we don't get our luxury items because of anarchy and lowered productivity in Uruguay. No, don't even hypothesize! My American privilege is crying already.
There are many different types of people on Earth. There are approximately 6,900 languages spoken in the world, and around 250 nationalities. How would this Utopian Earth even begin to mediate between these nationalities, or speak with them? (On an unrelated note: who would rule them? Would people eventually rise up?)
We would most likely have to separate these groups of people in order to avoid conflict, but this may lead to them feeling discriminated against, and eventual uprisings. We would have to be very careful, or there would be numerous wars and uprisings. We'd have to accommodate for each of their customs and traditions, and protect each from each other. If we treated any even remotely different than another, then we would seem cruel or biased, and they would think, " Hey we don't like how you treat us, so were gonna take over and treat us better." And then the Utopia is ruined.