I've seen the argument from those that mention "opt-in" in regards to Fediverse bridges that they didn't consent to having their posts on other platforms....

Isn't that what the heck we're here for?!? At least a good bit of us...It's how the platform was designed to be.

There are MANY other platforms which have that closed space you're looking for.

If you're discriminating against millions of users based on not using the same platform, Fediverse is definitely not for you.