A marketplace of apps is actually a good thing. Not everyone wants something based on Twitter (i.e. Mastodon, and similar). Not everyone wants Reddit (Lemmy, Mbin, etc.). Not everyone wants a traditional forum (NodeBB). Not everyone wants a blog (WordPress, Hubzilla, etc.). Not everyone wants Facebook (Friendica, Hubzilla, etc.).
One of the goals is to build increased compatibility between apps so that you can choose which experience you want, yet can still talk to anyone else on the fediverse. Some big players will certainly emerge, but I think that there will always be hundreds of compatible apps.
We never know what the future holds. But one of our goals is to decentralize social media. If we wind up with only a few major players, we have failed in that goal.