I think a lot of the issue is the actual term. Defederation sounds like a lofty thing that we're inflicting on a server. It's just a block. Like you block a person or community on this instance, they still can type messages and they're still on the instance but you can't see them.
If I'm running an instance then defederation is basically me choosing inserting a user onto your personal block list. You may like a certain type of humor and I think it's annoying. You may like Popping videos but I find them gross. I can choose, on my own to block those things and my blocking Popping videos or dead baby joke communities is my personal choice.
But if I chose to add those items to YOUR block list then suddenly I'm in the wrong. It isn't up to me to say you can't like Popping videos (even if I find them gross) and I can't tell you that you can't read those dead baby jokes that you really laugh at (even if I think they're offensive).
So why even allow a feature like defederation? Because there is some content that we ALL wouldn't mind having blocked. It's unanimous that nobody wants spam in their feed no matter their position on Popping videos or dead baby jokes. People don't want to see CSAM in their feed. Nobody wants to see random private data about people being posted in their feed. In THOSE, very limited, cases then the ability of the instance admin to add an item to your block list is a positive feature. You only need a small group of people (moderators and admins) to detect and block abusive material and their work is shared by every single person on the instance.
Instead we have people who are advocating that we use defederation to impose their personal (or their group's) viewpoint on every other person on the same instance. This would be like me using my power to block spam instances in order to decide that you can't watch those Popping videos that you love so much. Suddenly this formerly useful tool is now being by others to curate what you're allowed to see on social media.
As far as Facebook, I imagine a lot of people would want to see content on Facebook via Lemmy. There will be instances that don't de-federate and those instances will see most of the user growth because they offer users both Fediverse and Facebook content... any instances that block Facebook will simply have a slightly different Fediverse with less people and less content.
The average user simply doesn't care about joining the battle against the corporate overlords, they're looking for the app that lets them see funny videos the easiest. Having all of the motivated ideological users in their own isolated bubble will ensure that Meta's section of the Fediverse can more easily be taken over by EEE. Meta will be the only developer developing features for the version of ActivityPub that is used in their network and so it will likely be adopted faster. Not having people developing FOSS-versions of ActivityPub extensions, apps and tools that are directly competing with Meta will create friction for people who want to transition away from Meta services and ensure their continued market dominance.
Federate with them, develop better tools and features, and then take their users away. Providing a better social media is how you beat Meta.
TL;DR
- Federation isn't the tool for this kind of ideological splintering and;
- Not federating with Meta services will ensure that they get all of the benefit of having an open source protocol without any competition for their userbase.