Canada

Ok fine, but what happens if hosting your own server becomes inaccessible due to costs and media companies become the only ones being able to do so and take control over the fediverse?

If a lot of people start using the fediverse the costs could become too much even if media companies aren't involved. The costs aren't dependent on the media companies' involvement it's dependent on the number of users on the service. If too many people start using the fediverse right now it's already the case where it could become unsustainable and servers may shut down and we'd have to go back to reddit or wherever.

Though I think it's doubtful that will happen. Various costs keep decreasing, and if the number of users increase, the amount of ad revenue and/or donation revenue will scale with that. Also the number of people with the knowledge and resources to set up a server to host a portion of users would increase.

Besides having a corporation handling some of the load would in some ways decrease the costs to private individuals and small organizations hosting their sites. Sure there would be more content available to serve to their users but that would be a good thing isn't it? The various user interface interactions from the users of a corporate fediverse site would cost that corporation bandwidth costs.