I guess one of the biggest negatives is that governments will gradually lose control on all aspects of society. I am not sure what would be the way the society runs if one day, the government stops working. I don't think that would happen. I had theorize it.
Decentralization at the moment has some problems. First, many people still can't accept it as many grow up in a centralize world. My dad still doesn't really belive in atm machines. He always thinks that the money inside the bank would be stolen. I guess it takes a lot more time and maybe a decade to have this technology understood by the masses. Even my friends don't understand this technology well.
Criminal organizations usually find a way to use a legitimate front-end to transfer and buy things using bit coins or other cryptos and it would be harder to track. There isn't many experts in this field yet who can build software that is able to track these transactions.
As a citizen, what if one day the government hired skill programmers to built a blockchain monitor to monitor everything about everyone in the country? It would be scary isn't it. Imagine if the government is able to look at every single transaction.
There are however still many positive sides of decentralization but those highlighted are just some. The world now is getting smaller and smaller because of the connectivity created by the internet.