Even if they bought up a few instances, they would never have the ability to buy them all. Even if they did, you could just create more that are outside corporate control.

The Internet used to be federated like this, where you would connect to servers that hosted various services such as chat (IRC), instant messaging (XMPP), streaming (IceCast), file sharing (FTP), etc. It wasn't until the 2010s that corporations started to centralize popular protocols into their own proprietary standards.

All we can do is let corporations run themselves into the ground over and over until people get sick of it and use stable, federated services.

It's worth keeping in mind that the World Wide Web, which people often incorrectly refer to as "the Internet", is the biggest federated service in existence. Companies like AOL tried and failed to make their own walled garden version of The Web.

The goal of the fediverse is to do what The Web did, but for every service online. This is something that could take decades, but it is something we need to do to make sure the future Internet is resilent against corporations and governments.