People misunderstand decentralisation in practice. Computer types, when they talk about decentralisation, usually mean a relatively large number of homogeneous nodes acting as peers. Which is cool but not how decentralised systems work in practice out in the meaty reality we all live in.
In reality, we have an economy that quickly works out who is the best (say, Samuel is the best) at something (say, mining bitcoin) and routes most of the resources available for hashing to Samuel. The situation is still decentralised, because if Samuel stops being the best at calculating hashes then resources will be reallocated to someone else. But in the short term all the resources will go to Samuel and he could do a lot of damage to the network.
People keep thinking that because the economy picks out a few winners that means that the system is now locked in stone - that isn't the case. The economy is perfectly happy to change things up radically when the situation changes. At least until a government steps in and regulates the ability to change flexibly out of the system.