As the fediverse grows, there will inevitably be more centralized instances. Every big tech corp may want to start their own instance, similar to how most tech corps provide their own mail services.
There are millions of email service providers, but Gmail and Outlook are synonymous to email for a large amount of people.
Defederating with Meta and Tumblr is like Protonmail blocking every mail from Gmail. You just cripple yourself and make your instance useless.