I'll interpret this to mean "If the problem is that Threads owns the majority of the userbase, why didn't you complain about Twitter owning the majority of the userbase?"

I'll reply to that as: Mastodon users did. That's why they used Mastodon in the first place, because they felt too much power was controlled in a single entity, so they complained and moved.

In terms of actions to take, what power was there with Twitter that Mastodon users did not exert? With Threads, Mastodon server owners have the power to defederate and block Threads trying to intermingle with their userbase. With Twitter, Mastodon users were the ones with the power to publicly disclose their Mastodon account and tell users to follow them on there.

In each instance, Mastodon users are doing what they can to reduce corporate overlords from having power over as many people as possible. Even if Threads is more centralized because of other instances defederating with it, the overall reach of Meta is reduced.