What are some technologies or protocols that started off decentralized and later become centralized? I look at a few examples across a broad spectrum.
Email (more specifically, SMTP) is a decentralized protocol. Anyone can run an email server that sends and receives email. But why don't we all run our personalized email servers? Speaking as someone who did run their own email server in college – it is one thing to receive mail, but another to make sure your email is reliably delivered. That's why we rely on services like consumer services like Gmail or enterprise services like MailChimp to send our email for us. One estimate puts Gmail at 27% of all email opens (source).
Git is a decentralized version control system (DVCS). Most descriptions don't include the decentralized adjective anymore, but it was important to distinguish it from the previous generation of centralized version control systems (CVCS). Those systems required all edits to be send to a centralized service – meaning that only one person could "check out" a file at a time. DVCS let each developer work on their own copy – to be merged together at a later time. But DVCS didn't really take hold until a centralized hub for discovery, collaboration, and storage was built – GitHub.
Tor, an anonymous overlay network, used .onion links instead of traditional .com links. These links were long strings of random characters that were difficult to remember or find. So just like the traditional web, there existed a wiki called "The Hidden Wiki" with links to popular onion sites, and eventually search engines that indexed sites on the Tor network. Centralization often creates real value for consumers.
Other peer-to-peer services like Skype and Napster eventually found themselves replaced by centralized services like Zoom and Spotify. Even pirated content (transferred primarily peer-to-peer) anecdotally has declined in popularity since the rise of streaming content platforms and centralized cloud services.
Before the United States had a central bank, there were extremely high rates of bank failures and panics. Businesses lacked access to reliable credit. In 1907, a panic sent an already weak economy into a spiral of bank runs and subsequent failures. With many markets on the brink of collapse, businesses turned to J.P. Morgan and other financiers to personally guarantee and flood the market with capital to keep it afloat. A few years later, the Federal Reserve Bank would be created to take the role that J.P. Morgan had provided in 1907.
There have been numerous attempts at decentralized social networks – Diaspora as a decentralized Facebook alternative (2010) and Mastodon as a decentralized Twitter alternative (2016). Decentralized social networks lack the same level of network effects and identity management.
Even in cryptocurrency, where decentralization is a stated goal, some of the most successful projects have succeeded by relaxing the constraints on decentralization. Solana, one of the most popular networks that claims low transaction fees with high throughput, achieves that performance by using a small selection of validation nodes. Not to mention centralized coin distribution and governance.