AT&T was for a time the sole provider of telephone service throughout most of the United States. In the 80s it was broken up into competing companies due to anti-trust action.
Before the breakup long-distance calls cost about $1-2 a minute ($3-5 today). After the breakup, they dropped to a few cents a minute. That's one effect of decentralization everyone (alive then) understood. Totally revolutionized telephone-usage.
Obviously some aspects can be more efficiently centralized. What matters is who controls the centralized part(s). E.g. There are still a few things to be re-learned from Usenet and Fido.
It's a good thing this has lead to an increase in competition, especially local competition, and not the entire marketing being owned by a few companies instead of one and none of them treading on each other's monopolized turf.
Arguably, we're actually conversing on a "decentralized" medium right now. HN may be popular in the tech scene but it's "just" a website. There are many "just" a websites out there!
Ultimately, that's what gets me about all these sorts of conversations. People will post articles on websites saying the web is dead or whatever…same thing happened 20 years ago, same thing will happen 20 years from now.
When has any medium gone from centralized → decentralized?