tl;dr Large, centralised networks are better for data collection and advertising than smaller, decentralised ones.
Centralisation versus decentralisation should not be an either/or style debate.
It is possible to have both types of networks. Each can be valuable in different contexts.
For example, we need to have networks for noncommercial purposes as well as commercial ones. Small networks, e.g., L2/3 overlays, managed by their users can be useful for noncommercial purposes. Large networks managed by central third party authorities can be useful for commercial purposes.
With respect to noncommercial internetworks, one lesson of The Internet is that large scales ones are inherently disadvantaged. Besides the problem of proprietary, non-compatible protocols mentioned in the top comment, centralised "infinite scale" networks also become infested with "tech" companies, commercial intermediaries, who usurp them as honeypots for commercial and political advertisers.^1
On "The Internet", noncommercial network use, e.g., communicating with friends and family, is being compromised for commercial purposes by a relatively small number of single, central authority websites. Hundreds of millions of people with small networks of friends and family, e.g., less than 100 nodes, are being unecessarily intermediated by central "hubs" that collect data and serve targeted advertising and/or sell advertising services.
If those smaller networks were not connected nor managed by a third parties, then the attraction of injecting advertising into peoples' noncommercial internet use falls dramatically. The incentive for, and feasibiity of, data collection is similarly reduced. Those are features, not bugs.
Whereas for commercial uses, e.g., sales of goods and services, inter-business communications, etc., small networks are far less valuable. For these applications, we need networks that scale to large sizes. Arguably that utility has already been proven as successful. Advertising and data collection may be appropriate in the commercial context.
1. The irony is that The Internet was supposed to be a means of disintermediation. In some contexts, that is true. In others, it has allowed for the greatest level of intermediation in history.