Old-design parts may not be available at all, or only at great expense. These and other factors add more and more to the cost of parts and installation at every step of the maintenance chain. The number of insured accidents is up. Now, it's very unusual for any airplane to fly without hull insurance. The value of airplanes is soaring.
More valuable airplanes mean more expensive repair bills.
Damage that twenty years ago would be fixed cheaply with plentiful parts now would cost a significant percentage of the insured value of the airplane to repair. For a decade our insurance rates were artificially low because insurance companies could take the premiums and invest them in the stock market. Insurance company boards will no longer accept the possibility of market gains as an explanation for keeping insurance premiums low. The ever-present threat of lawsuits scares underwriters. It's simply too easy to prove pilot negligence, because most aviation accidents do in fact result from pilot error.