People on or entering the fediverse understand the variety of available options.
If someone isn't aware that they're on the fediverse, then they can't really benefit from the openness and customizability that it provides. A mastodon.social user who knows nothing of the fediverse won't know that they can move to a different Mastodon instance or interact with the same content using Friendica, as they won't know that the options exist to begin with.
Furthermore, people will have more incentive to preserve an open fediverse if they're aware that it exists. If the fediverse is filled with people who, for example, think that Threads is all there is or didn't come to Threads with an awareness of the fediverse, the fediverse becomes much easier to undermine.
There is no downside to using free and open-source platforms over proprietary ones.
If someone wants to join a closed-source instance run by a for-profit company, they should absolutely be able to. However, that should ideally be because they prefer an instance moderated by Meta, not because the free and open-source alternatives are relatively lacking. Open-source software is extremely important in order for users to have options and agency, so we should aim for these factors to not come with a sacrifice. Otherwise, companies will be able to draw most newcomers to their instance and attain a large share of the content on the fediverse, which is bad as discussed with Statement #1.
Going by this principle, if the owner over a closed-source fediverse platform starts trying to create exclusive functionality that would attract people their instance, they should be regarded with extreme caution. If you're familiar with the whole "Embrace, Extend, Extinguish" thing, a company doing such would be the "Extend" phase of EEE, and that's a situation we should avoid at all costs.