A layer zero type protocol that will allow other projects to join. The biggest blockchain competitor to YouTube by a mile is LBRY / Odysee. That could integrate into the ATProtocol and host on BlueSky / Twitter. And then Twitter can bootstrap all of the other competitors that join on chain. And people could freely move from one protocol to the other seamlessly through a single account they control web3 style.
I think you have broad imaginations filled with optimism about the future of YouTube and Facebook. Do you really think that it is easy to establish a simulation system capable of competing with these two giants? Do you have an understanding of how much this will cost and how long it will take? Of course, taking into account that both companies will not take any step to limit or prevent this from happening.
Elon Musk's priority when putting $44 billion to complete the deal cannot be to destroy his rivals in the most sophisticated way. There is a lot that can be done and a lot of profits that can be reaped at different levels, including in the political arena, which lags behind the management of politicians on social media platforms.