I think renovating planes to new would be a bit too much. It would effectively be allowing airlines to get free planes in exchange for a worthless scrap. In real life, old planes are only attractive to cargo operators because they are near-worthless for passenger service. I think lower maintenance costs (or making them less age-dependent) on cargo aircraft would be more appropriate - so that they are economical to run in their old age.
That said, it is a bit false to make maintenance costs cheaper - in real life it's not so much the lower running costs (indeed, the older planes may be more costly to run) as the fact that they are costly to overhaul/modernise (for passengers) and that they provide a more likely return on investment (i.e. minimal investment, making them easier to profit on).
Those sort of factors are less of an issue in AM, in the former because plane age has no effect on passenger revenue, and in the latter because you are basically guaranteed revenue on anything other than the busiest routes.
