Do you think kids wouldn't develop the same mental health issues if they were on Mastodon instead of Facebook?
Do you think the largest Mastodon instances wouldn't start also harming their users once large enough?
There's nothing preventing the decentralized from becoming centralized over time, from the same network effects, and abusing the users all over again.
In which case, why fight for it? It's pointless; we started decentralized and became centralized; a re-decentralization moment just causes the same economic forces to pull everything together again.