Choosing between repairing original wood windows and replacing only the worst ones

Our house still has several original wooden windows. Some have minor rot at the lower corners and others are in reasonable condition but drafty. Replacing all of them would be expensive and change the character of the rooms. I'm weighing targeted wood repairs, weather stripping and secondary glazing against replacing only the worst frames. What condition would make you stop repairing an old window and accept that replacement is the responsible choice?