I understand what you are saying Rammel, and if you are correct I have to agree. The root core of the problem is that cost accounting is an approximate science and attempts to model something like an airline in detail will ultimately come down to the question of the assumptions you make to allocate fixed and indirect costs, because the 'profit" of business units within the entity is illusory.
What usually happens in large businesses that decide to go down this road, unless they are very well managed, is that there is a wild scramble by middle managers to shift costs onto some other part of the business.
The net result, as I know to my cost
, is a giant management s**t fight, nobody looking after the business, and the most bizarre middle management decisions being taken in an effort to make their divisions look good. This normally results in INCREASED costs. Its like holding a rubber tube underwater, you push it down in one place, it pops up somewhere else.
For example, Supply decides to cut inventory of part XYZ, so now the old XYZ has to be refurbished and reused, supply has reduced its costs, but the workshops costs have just increased.
Sorry for the rant, I had a few years of dealing with this sort of thing, I still remember F28 APU starters at AN, I think we went through about 60 of them before somebody kicked Garrett in the nuts hard enough to fix the clutch problem.