They won't fragment it. They'll absorb its content while contributing as little as possible to it. Here's how it will happen.
They'll extend the feature set in a way that makes their instance more desirable to use. They'll have boosts, but they'll also have private systems to engage with like Reddit karma or Slashdot-style voting (funny, informative, insightful, etc). Anything that engages your stupid lizard brain to make the chemicals, and even better if those systems just aren't supported by ActivityPub because they have plausible deniability for why these features only work on their thing.
Having created The Nicest Fediverse Server Ever that, so far, appears to be playing by all the rules, they'll begin to dominate. A few voices will protest, but most will say "no, this seems to all be above board!" And because that will be true at the time, our Prophets Cassandra will be largely ignored. We'll stay at this point until The Nicest Fediverse Server Ever has a majority of Fediverse users registered there and using it as their primary portal to the Fediverse. This is the most likely point to introduce advertising, when user trust is high and they can claim that they need to support the business of hosting all this. If they offer subscriptions, they won't be necessary to access the site yet.
After capturing a critical mass of users, they'll become covertly, then overtly, hostile toward other federated servers. They'll allow you to participate in a thread from a federated server and dutifully share your post so you can enjoy getting replies, but they'll have private threads too, until the private ones outnumber the federated ones. They'll add more features that are either incompatible with ActivityPub or simply not shared with it, like a privately owned Imgur that only works with their site, or choosing to encrypt some part of user communication so it's not practical to interoperate. They'll defederate from some servers because THINK OF THE CHILDREN, some because they're poorly managed, and some, in time, for not being Advertiser-Friendly. The ads now go fully rancid, the gloves come off, and would you look at that: your data HAS been sold to the highest bidder all this time.
Now fully enshittified, they'll declare that the majority of their users are no longer interested in federation and that doing so is an expense they don't need. They will defederate entirely, keep all the users, and kill everything that came before it.