16 years is not too bad, even the exceptions of Soviet/Russian metal can continue in use. A D-Check should only be undertaken at 16 years if you are going to use that aircraft until at least 20 years, ideally until the full 8 until 24. D and C checks are added to book value and depreciate, so in cash terms are annoying at the time of payment, but in accounting terms are considered Non-Current assets, so they should be treated as such and your Balance Sheet won't change in value because of them.
Not forgetting new aircraft are more expensive. Lets say in 2000 you have A320CEO for 75million and in 2016 you have the option to swap it for a A320NEO for 120million. The NEO will have higher ownership costs in terms of depreciation, so will effectively yield less profit from swapping, even taking into account fuel efficiency savings. So at that point continuing with the 2nd D-Check at 16 years can make a lot of sense, your aircraft has still got plenty of life, save the big Capital expenditure until when you absolutely need it, or use it for expansion.
It's a waste of money to replace aircraft early if you a just going to end up scrapping them when keeping them running for another 8 years is often a better option. In large fleets changeovers, that's when 16 years scrapping often makes the most sense, as you usually aim to have these done in a very short space of time, your oldest could be nearly 24 and the youngest around 16 when swapping them out, at that point scrapping 16 year old aircraft is often the better option, as usually the fleet you are replacing is outdated and will be hard to sell on the market.