If it ends up that meta is able to destroy the fediverse simply by joining it, that is a design flaw on OUR end.
“Simply by joining it” is not an accurate representation of what will happen in the slightest. Meta is not some scrappy little Lemmy instance operator relying on donations to keep the lights on, they’re one of the biggest companies in the world who simply do not care about fair competition or open standards, and they have a proven track record of using that position to either buy out or destroy competition.
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? You can make a project as resistant to corporate overreach as you like, infrastructure to run it still costs money and there is no fediverse operator on the face of the earth that is going to be able to outspend Meta when it comes to infrastructure and R&D. How is defederation not an appropriate response when smaller instances are crippled under the inevitable load stemming from Metas users?
Corporations have been embracing, extending and extinguishing FOSS projects in the tech space for decades now, and their demise has rarely been because of a fatal flaw in the projects themselves. It’s been an intentional play by Microsoft, Google et al to ensure that there is no viable open alternative to their walled gardens. Trusting them in any capacity is naïve at best and catastrophic at worst.
I encourage you to read this blog post which outlines these concerns much better than I can: https://ploum.net/2023-06-23-how-to-kill-decentralised-networks.html