Personally, I don't think replacement should be the goal. As others have said, a better, more likely outcome is that the Fediverse become a viable alternative to big social media (in the eyes of the public) & an influential part of the ecosystem.
And anyways, the Fediverse is a solution for me - and others, presumably - but it probably won't be the solution for social media influencers, terminally online political provocateurs, people stuck in the endless, algorithmic (and psychologically manipulative) stream of 'content' which Big Social offers, advertisers, etc. As long as people have those sorts of relationships to social media and as long as capitalism and consumerism exist, big social media platforms will always be around to capitalize on that and fill that niche.
Plus, the barrier to entry for the Fediverse is technical skill, which has impeded its accessibility to the broader public. While making the Fediverse more accessible and cultivating that technical skill and know-how in the public are both things I support, I appreciate the more intentional social media communities which are forming around here and are able to grow sustainably quite possibly because it's harder for the average person to wrap their head around. It reminds me a lot of the older days of the internet.
The best thing for that, IMO, is for the Fediverse to continually exist in its decentralized state and provide unique examples of how social media could be, for it to keep growing slowly, for average people to come here of their own volition, see how things are here, and decide on their own that they want to be part of it.
BreadDog