When you take out the insurance on a used one you agree the hull value with the insurer. As the machine ages, the hull value indeed does go down, so if you agree a new hull value each year then your hull premium element should indeed fall. However, in the UK at least, there are break points in value where the percentage premium rate charged falls as the hull value increases. So you may well find that a lower agreed hull value is charged at a higher % rate so the premium is actually greater.
The biggest risk in doing what you are proposing is that training ships do get bashed about with repeated EOLs etc. Hence the maintenance costs may well be a lot higher than you are budgeting for. remember, you can't claim for everything that might happen on insurance.