The route won't become a fee run.
We will still have to pay the usual charges.
So where do you think the money will be going then? Surely not to NATS because you say thay won't need it because they will have sacked loads of people and ramped up domestic enroute charges. The exact financial details of the handover between NATS and IAA may include the IAA effectively paying NATS for a period but that is up to them.
The IAA have the radar, the personnel, the communications and the capacity.
Did I hear the London people crying when ScACC took over their sectors. No. Perhaps they were happy that safety and the customer comes first.
Regards,
DFC