In aviation, things work very differently. The replacement value of an airplane doesn't follow your typical depreciation scale. Even a Cessna 172 sold for $20k back in 1975 might be worth $45k today.
So in aviation, instead of using Actual Cash Value, the payout is based on a fixed Agreed Value. I pay $X dollars premium, and the insurance company agrees to pay me $Y dollars if there's a loss. There is no depreciation involved.
This is exactly my experience with insuring my own aircraft (a 1979 Beech Sundowner 180). Although the market for GA aircraft has tanked in my part of the world and the Agreed Value has declined.
The caveat here is that the insurance company will balk if you ask to insure for an Agreed Value greatly above the true market value of the aircraft. They don't want to pay out for much greater than true market value (at my end of the scale the premium difference on a $10k delta in agreed value is fairly small). And of course I don't want to pay excessive premiums, so it's in both of our best interests to assess a fair Agreed Value.
Two things have affected the Agreed Value on my aircraft: one, the market (which has been pretty poor since 2008); the other, as I run down the time to TBO on my engine.
While I don't know if I can extrapolate this to airliners, I'm pretty sure that as an aircraft approaches a major check, it's market and hence agreed value adjusts accordingly. Since I bought my aircraft, I've reduced the agreed value as my engine approaches TBO.