I have never understood your theory that an old plane is expensive to maintain because of airframe parts.
Obviously that's not true, so it could not have been a theory of mine.
It is the accumulated cost of poor maintenance (specifically poor lubrication, and corrosion) which makes an old plane potentially much more expensive to bring up to a "perfect" standard after it is bought by the next punter.
There are obviously exceptions, depending on whether the previous owner was diligent or not, but most owners do not maintain on a money no object basis. Most let things wear out and then they flog the plane. Same with cars.
But firstly few people are going to "totally fix" a 30 year old plane they have just bought, so they will be running along with a part of the legacy they bought, and secondly corrosion is something that does get increasingly expensive with airframe age (relative to hangarage history obviously, etc). That is just actual practice out there.
I have owned 2 old twins and the airframe parts cost has been negligible
After the total-stripdown rebuild you had done on your 421C (ballpark cost £100k plus avionics?) I would have expected it to be close to zero for the rest of your flying life
Re the 80% fuel burn overhead, what figure would you offer?