Do you wait until your outside air conditioning unit dies, or do you go ahead and replace it knowing it is on its last leg?

My air conditioner broke down again this week, during 105 degree temperatures of course, and my a/c guy told me again the unit is on its last leg. Luckily it was the capacitor and was only a 20.00 item. This unit will be 20 years old next year. Do I go ahead and bite the bullet and get a new one now or do I wait until it has actually died? Reasons for either side will be helpful.