I imagine serious cyclists also carry spare spokes, spare brake pads, spare derailleur hangers, spare bearings, spare cables, spare tyres, lubricant, assembly grease, spare batteries, spanner for truing bent components, hydraulic fluid (if using disc brakes), spare helmet in event of crash, etc.

The worst mechanical I ever had was a rear derailleur which deformed under load and pulled into wheel. I have never heard of anyone carrying a complete spare derailleur unless on a round the world trip.