Just paying recurring fees (e.g. paying rent to your landlord) does not match the definition of rent-seeking.
Operations spending is required, but does not generate any long term wealth. You keep cleaning a hotel ever day and it does not get ever cleaner over time the way an actual investment does when you keep investing more money.
operators of a domain are essentially stewards of that domain and ensure that it's operated correctly, that there are no duplicates, and that it can be trusted. They do create value in the same way the American bar association does, by establishing and maintaining trust between end-users and providers of a service.
People do not expect the new owners will operate the domain correctly, meaning ensure that the TLD remains available at reasonable prices and conditions to the current types of users.
It's textbook economic rent.