The Fediverse is bound to come to the attention of big corporations, and if it becomes big enough they will view it as competition and try to crush it. I doubt it can outcompete them in terms of popularity. The best hope has to be coexistence, in which the Fediverse doesn't try to win the most users, but defends its integrity against large corporations entering the space to sabotage it.
The comparison with XMPP may not be conclusive: XMPP is purely a communication protocol, so if not many people use it, not many people can be reached through it and it becomes less useful. Something like Lemmy, by contrast, is not intended to get you in touch with everyone in the world. It can function as long as it has enough users to make it interesting, enough money to pay for servers, and enough skilled developers willing to work on it. It doesn't need huge numbers of users, and it doesn't need to outcompete Reddit or any other corporate platform.
I hope Lemmy can equip itself with good tools for managing trolls and other kinds of attack, including corporate-led sabotage, because those things are likely to come soon. There has been an explosion in bot accounts recently, which are ominously dormant for the time being. If those all get switched on at once, there will be a huge amount of noise and a big increase in traffic. Lemmy needs to prioritize equipping itself to withstand this.
But if Meta enters the space and siphons off a bunch of users, there's no reason the rest of us can't continue here as before, without it. It may be a relatively small community but it can still function.
NotTheOnlyGamer