Defederation would be more akin to a country cutting ties with another in international relationships, when this other country keeps causing trouble, or when making concessions for its problems makes the problems spill over or the bureaucratic cost to become too high. There's still, in this analogy, another 200-ish countries to have relationships with.

In the oxygenating analogy, instead of just one waterwheel, oxygen-blowing tube, small waterfall, or the sort, it's multiple ones. And if one source starts pumping CO2, motor oil, sucking up liquid instead of giving oxygen/oxygenated air to the aquarium, or whatever, this source can be cut without stopping the oxygenating.

And the nature of the protocol and the services that use it also allow people to come over from elsewhere. What I meant, far from "creating users", is to open venues which people can find the platforms of the network, and for the people already in the network to have access to places otherwise barren, and that by interacting with, they can bump into relevance.

On linking externally, from my experience, it's not uncommon for example in places like Pixiv for creators to link their socials, fediverse ones like Mastodon and Misskey included. You can also connect to other protocols such as Bluesky through bridges. Commercial projects such as Threads and Wordpress also have optional ActivityPub integration which would allow more people to join in or at least be aware of. And the services using the protocol are also niche-based but rarely alienating their userbases regarding other places in the network.

And on bigger forums having a larger audience, they usually do so by being so big they cannibalize competition (even if potentially not intentionally) while keeping their userbases in a walled garden. But when alternatives die out, and what's left enshittify, people have nowhere to run. But when Reddit started going from bad to worse, a lot of people found here as an alternative. Similarly, Twitter's at least kinda shitty since always, and very few altenatives stuck around, Mastodon being one of them.

And from what I can notice, the centralizing platforms, even the bigger ones, are either shrinking (e.g. Reddit), already shrunk (e.g. Facebook), or show indications that they may be shrinking (e.g. Discord allowing search engine indexing, and Meta allowing to sign up on Threads with their other services). And places that stay strong despite being centralizing usually offer more than just the forum experience or adjacents.

But once more, unless I've missed it, defederation is very sporadic. And when used, it is usually done so to keep the instance healthy within their culture.