This doesn't really go back in time much. What about Usenet? And why did it die?
Last mile was always in the hands of oligopolies, if not monopolies. In New York, Andrew Cuomo (currently governor) strong-armed the last mile carriers into blocking Usenet for "think of the children" reasons. So how was the peer to peer Usenet killed off? Monopoly big business and big government.
These things don't exist in a vacuum - big business and the government work together to kill off decentralization and work towards centralization. Cuomo is a Democrat, but someone like Mitch McConnell is no different.
> My statement that email can be used without involving Google
I run my own mail server, and have kept compliant with what they want in terms of SPF and whatnot. They have an influence, but if they made Gmail a complete walled garden it would be self-defeating at this point.
Linux ran for years via e-mail and patch - huge Linux companies were IPO'ing in the 1990s and Linus was still using e-mails and patches to keep the system together.
I also still communicate via IRC. I'm sure a decentralized Slack-like IRC could be done if anyone cared to do so.
Git is a good example - it's a thing that is decentralized newer than other things mentioned.
Decentralization works just fine for me. Guess what decentralized in 2019? IT being centered in San Francisco.
IBM (and the seven dwarves) dominated centralized computing until the MOS microcomputers started selling like hotcakes in the 1970s. I have an Android phone in my pocket, and have futzed around with LineageOS (although did not put it on my phone yet). Who knew in 1974 that people could walk around with phones/computers in their pocket that was many times more powerful that an IBM S/360, that the people would have the source code to and could modify at will? That's a decentralization you aren't even aware was a decentralization if you're too young to have heard of white shirted IBM technicians shuffling punch cards.