I think an aviation week article, back when the slot swap had just been denied, said that with Delta suing the government then for the first time slots would go to court and the question of who they really belong to would be settled. Delta said the owned them and the FAA said they did.

So now that a deal was reached then both sides got a push rather than a potentially devastating loss of control.

I think even if the slots could revert back to the FAA you could claim that's a part of the deal to prevent a antitrust suit.

But I think the airlines own them due to their history and nature of their origination but it'd been interesting to let a court decide.