So admins shouldn’t defederate from any instances at all? Even right wing Nazi instances with Nazi flags in every profile?

Users should have the power to defederate in addition to admins.

This is something the people who join those instances want.

Not everyone wants the same thing or has the same idea of what constitutes 'harmful' instances.

They also want to “deplatform” those types from society as a whole.

Yes, this is why it's important to take individual control away from the user; to push an agenda.