Money wins, unless the tech is done well.
It would be good, but not for the bottom line. Facebook has mastered the bottom line indeed.
The point in more users leading to a healthy revenue wouldn't be to support a profit-seeking company—something that many users are specifically here to avoid—but rather to ensure the instance is cushioned against unforeseen events and cost increases. Instances that can be expected to last beyond the short-term to medium-term should be backed by a non-profit organization with commitments to users to remove key man risk.
Even under the scenario of full data portability, switching instances creates confusion (particularly for casual users who just want to create an account and not worry about whether it will still exist in a year), eliminates the archive of content on the shuttered instance that will no longer federate, and breaks @ mention links, so safeguards to reduce the risk of instance failure outweigh the downsides of reduced decentralization.
Given that the software is being developed by literal communists and isn't in the hands of the Lemmy.world operators, I don't think limited centralization is likely to lead to user base exploitation or a particularly strong network effect.