What actually ends a system's life
Rarely old age alone. Furnaces die by heat exchanger corrosion — accelerated in our damp coastal air. Heat pumps and ACs die by compressor failure, usually after years of running with airflow restrictions or low charge. Tank water heaters die by tank corrosion once the anode rod is consumed — a part that costs little to replace and almost nobody replaces. Every one of these failure modes is slowed by routine maintenance.
The repair-or-replace math
A useful rule: multiply the repair quote by the equipment's age in years; past roughly 5,000, replacement deserves a serious look. A $600 repair on an 8-year-old furnace (4,800) is reasonable; the same repair at 15 years (9,000) is money toward a system already near end-of-life. Add two more factors: efficiency gains — a modern heat pump can cut heating energy dramatically versus old electric or oil heat — and rebates, which only apply to replacement, never repair.
End-of-life is a planning opportunity
Replacement under emergency conditions is the worst way to buy HVAC: no time for load calculations, limited equipment availability, peak-season pricing. If your system is past 12–15 years, have its condition assessed during a routine service and plan the replacement on your schedule. That's also the moment to re-run the heat loss / heat gain numbers — homes change (insulation, windows, additions) and the right size today may not be what was installed twenty years ago.