Think about the headache that would be had if to send an email, the domain name had to be the same.

Think about all the headaches we have now because anyone with a domain can send spam emails to you. Email would have been much better if there were only a few, or at best, a single email provider. In fact, the reason why email is usable today is because we don't treat all domains as equal and put more trust in a few providers. The concentration of email users into a few domains is what allows it to be used by so many people today.

If you want to live in your own instance, atleast with lemmy, you can just never leave the local tab,

But users from other instances can still interact with local posts.

as for Lemmygrad pushing to de-federate, dieing out, or pushing for lemmy or other instinces to purge users, I have not seen that, the closest I have seen is a “this is why purges are necessary” and that is a comment ment for the in group, as lemmy.ml mentions they are a leftist website.

I didn't mean to single out any particular incident or group of users. I was attempting to make the point that federation is a major roadblock to the ability of every group of people to block "outsiders." Other important needs of people are also hampered by federation (for example, discoverability).