Ah, yeah, true, I still remember the days when the walled garden or IRC was the only option around before XMPP was invented.

Are you sure you aren't just saying "those decentralized services that happen to be built as replacements for centralized services come out later than what they intend to replace"? Well, yeah, of course they do. While those that happen to be invented decentralized first come out decentralized first. Also, I think email still is a decentralized service of today? And yet, gmail seems to be going strong in centralizing email, doesn't it?!