Well, I think the "promise" was to replace fiat in a low cost and fast p2p payment system. That has failed to materialize in any meaningful way. Don't get me wrong, it still can happen. Just that it has not happened yet and the general public has not embraced bitcoin as a fiat replacement.
Decentralized just means that no one entity has control or large influence on the network. An entity could be a government, a company, a group, or a single person.