I have asked that question too in the past, but I have since adopted a more nuanced perspective as to the answer I would give. Was Facebook ready for primetime in its early days? Or Twitter, Google Search, Github? The entire early web? When was the theshold crossed, and how much was luck involved and the unique circumstances that allowed that to happen? Why did some make it and so many others not?
And then consider “primetime”. What does it even mean? Ready for billions of people to adopt? Or readiness for production-use? If the latter, then fediverse has well reached that goal.
If you want to make a popular social networking application, then there’s nothing in ActivityPub et al that withholds you from doing so. Whether your app reaches its ambitious goals is probably more dependent on many other factors, such as user experience, productization, PR and marketing. Your app should be able to reach great heights while not even interoperating with other apps, and giving you full freedom to how you extend the protocol.
What is unique to ActivityPub and has never truly existed before, is that the standards offer social primitives that allow much broader interoperability between a wide variety of different apps. This is where the frontier of discovery is, and things are still very immature. But should you be a true early adopter of the broader vision and untapped potential of the federated social fabric that might possibly arise, then there’s good chance you be one of the players that is able to directly influence direction in desirable ways as well. Just like Mastodon is able to right now, in the microblogging space.