I'm slightly puzzled as to why people are so interested in the cost of depreciation when comparing a B747 v B777 v A380. The capital cost of the aircraft is a sunk cost - asking Toulouse if you can send the aircraft back to the factory in return for a refund of 50% because you've used it for only 50% of the tax authority permitted depreciation period isn't gonna work.
Depreciation is a non-cash accounting charge - it just spreads the accounting recognition of the capital cost over a number of years, so that the company accounts give a fair reflection to shareholders as to how the firm is doing and allowing comparison between different years. The actual capital cost of the aircraft / engines is paid on terms determined by the manufacturer - not the same as IFRS or other accounting rules. Whether an aircraft is spending 12 hours a day up in the air, or 24 hours a day in the desert, Airbus, Boeing, ILFC or anyone else still want their money and will file in court to get their airframe back if money owed is heavily overdue with regards to payment terms.
In assessing whether to continue operating a particular aircraft, would cashflow make more sense as a determinant ? Compare the cash that can be obtained (if any) by leasing an aircraft out with the revenue from ticket sales and the cost of fuel, crew and all other operational matters. Of course, putting an old aircraft into desert storage is an option in case the economics look better a few years in the future.
Delta's / Northwest's DC9s were fully depreciated many many years ago, but they were pensioned off a few years ago for a reason...