Earlier quoted context omitted.
You're forgetting spam. Spam destroyed all those first generation federated systems. IRC survived because it was too niche for spammers to target much but spam is the primary thing that killed Usenet and email as a truly open system. The closed systems were better able to fight spam because they could easily ban people and IPs. On a deeper level spam, "brattish" commercial sites, etc. all come from when money got inv…
So are you saying that you believe YouTube will also perish because it's now mostly spam, albeit sanctioned spam? Not a facetious comment, I genuinely wonder. It'd be a wonderful twist, in a way.
The systems that died were systems that were structurally unable to fight spam or where doing so was prohibitively expensive in time or money.