I am not so worried about a few big tech companies embracing fediverse, because if a couple of them do, it may draw the even bigger fish in too, meaning that for the first time we will have interoperability between major social media companies.
If we like the unique feel of Tumblr, the rapidity of Twitter, the artistic community of deviantart or the targeted boosts offered by Facebook, etc. we could choose one of those services knowing that we can still them to stay in touch with our friends on other networks.
That will still not be enough to persuade many of us to join those commercial networks, but we will finally be able to read posts sent by our friends, and they will be able to read ours. If that happens, it will be great, because it will no longer matter what service people decide to join.
Even the big companies may eventually see that interoperability is to their advantage – they will simply need to shift their attention away from all kinds of devious behaviour that aims to lock in users by force, and towards offering the best experience possible. When people are no longer held captive, they will be able to demand more.
If I already enjoy being on Facebook, but can also see there all the posts of my friends on Twitter, I will end up spending more of my time on Facebook, which is eventually better for Facebook.
The only real danger, as far as I understand it, is that as with email, it could become a playing field mainly of a few big operators. Because in order for it to work properly, there is quite a high bar to reach. It has to be done with a slew of protocols and security standards. But new email companies and services, even non-profits run by a few volunteers, do manage to break in, and even manage to be innovative in what they offer.
I think this will be the same with the fediverse. What will eventually persuade the big companies to open up and be interoperable will be government regulations or other necessities, rather than “competition” from Mastodon. But the availability of common free opensource protocols like ActivityPub is showing the way forward.