if depreciation equals the monthly lease, a plane would be worth nothing after 4 years. If you then sell a four year old 73G for 10 million, your account will tell you that you made a 10 million profit by selling that plane. With the current depreciation I can sell a two/three year old 73G for about 34 million (if I find a buyer) and my accountant tells me that I lost 8 million by selling that airplane.
Whatever my accountant tells me, I know I rather get 34 million than 10 million ;-)
At this moment a new 73G depreciates roughly 2 million per year. If after two years you sell that plane for 42 million, your accountant will not complain. It means you will have sold the plane at book value, so you made no loss.
The problem is not that depreciation goes too slowly. Problem is that there is nobody buys your plane if you ask 42 million. If I put a two/three year old 73G on the market for 34 million, that means I sell it at 80% of the book value. My accountant tells me I loose 8 million because I ask less than the book value of 42 million.
If I want to keep planes without paying extra staff for my fleet and network, I put them up for sale on the second hand market at 100%. Never has someone bought a second hand plane at the book value. I repeat, never. We're talking about buying, not leasing. If you want to sell a plane, you simply have to dive under the real value of the plane. You can sell a plane at book value when it is 10 years or older. And if you buy a 25 year old 735 for a few millions, you can probably even sell it with a profit
There are a few reasons...
AS sells second hand planes under the book value, so the game competes with you.
Return on investment is much higher if you lease planes, so a lot of people never buy.
Players know you have no alternative. If they don't buy it, you have to "give" the plane to AS for 10% (well... 15%) of the value.
By the way, you cannot sell a plane at book value. If you ask the real value, it will be on the market at 105% of the book value.
And I don't think higher depreciation rate would improve things... the result would be that AS sells second hand planes much cheaper than today. You would still have to compete with them :-)
It would also make it a lot cheaper to lease used planes. So you would increase overall growth rate on a server.
So all in all, I don't think that just increasing depreciation rate would solve your problem.
However, I agree that 20 years (or 24 years) is a long time. Depreciation may be calculated in real time, profits are surely not in real time.