Our San Marcos system is over 40 years old and still runs. Should we replace it before it fails?

The market's own record shows units past 40 years still in service here, so yours is unusual but not unique, and the case for planned replacement is the calendar, not fear. A system that old fails on its own schedule, which in this city means during settled inland heat, at emergency pricing, with the household choosing equipment under duress from whatever is on a truck. Planned replacement in spring or fall buys measured sizing, duct corrections, and compared quotes. The honest exception: if the unit is lightly used, a mild exposure home, a shaded valley floor lot, a staged plan with a defined failure trigger can be rational. What is not rational is the third major repair in two summers at this age; that money is a down payment spent twice.