If defederation worked more like a shadowban, I would agree with you. .world defederating with feddit.nl could result in no one from .world seeing any content posted by anyone from .nl, but .nl can still see the content from .world. It’s unfortunate that .nl couldn’t participate anymore, but it’s better than that community ceasing to exist.
Defederations as they exist now are like being in a restaurant. There are two employees, one makes the food, the other makes the drinks. It starts out with being able to order either, or both, right from the comfort of your table. However, the owner of the restaurant decides that he just doesn’t care much for drinks, and kicks out the drink-maker. The drink-maker opens a new store across the street, but he does not serve food, only drinks, and the restaurant you started at doesn’t serve drinks, only food. Yes, technically, you are still free to go across the street to get some drinks, or stay here for some food, but boy, wouldn’t it be nicer if that owner never decided he didn’t like drinks? Sure, not EVERYONE in the restaurant cares, but a pretty significant amount of them probably would. They might just go down the street to McReddits. Sure, the food there is pretty bad, but at least they can get both food and drinks without having to go across the street.