When Meta have so much money that they can simply outspend any other fediverse platform and become dominant that way, how is that a design flaw on our end?
It's a design flaw when simply "outspending" other fediverse platforms allows you to dominate them. There are ways to design a system where that's not possible.
The fact that those other fediverse platforms can defederate from the "big money instance" if they don't like what it's doing, for example, is a part of the design of the fediverse that can help counter influence like that. You can't force other instances to federate with you even if you have an enormous amount of money, and even if you did manage it in specific cases other people without that vulnerability can just spin up new instances.
We'll see whether this sort of thing is "enough", I guess, because Meta is coming one way or the other. If it turns out that stronger defenses are needed then there are other technical methods that could be used to strengthen the decentralized nature of the fediverse.