Defederation is a double-edged sword. In the end, those who defederate will lock themselves into a smaller space and lose out on content produced by users outside of their instance. With how hard it currently is to discover content, a big instance defederating from lots of other instances with little good reason can easily backfire. At least that's how I see it from what I currently understand about Lemmy.
I think people are trying to defederate to filter content, which is not something that defederating is good for right now, cause every big instance has a lot of diverse communities. Also, there's been concerns about bots and safety which makes sense, but that doesn't mean that we're under risk of staying defederated permanently or for any significant amount of time. Moderation tools, content filtering and discovery needs to become much better before that resolved for everyone in a satisfying way.
As for /c/thedonald and communities like it specifically, it's ridiculous to take what seems to me as satire as 100% serious straight up bigotry, but I do understand that we've seen communities with origins in satire turn to serious on the internet before. I think an instance-wide flag for communities that signals that the content is satire when you open them or see a post from them. Allow communities to be created with it from the start but only modifiable by admins afterwards, so you can't take it away or add it after a community is made. It might just work to mitigate this problem and help punish rule breaking quicker and easier.