This week, we've replaced the heart and lungs of our house - furnace, air conditioner, and water heater. None of these were broken, per se. The hot water tank was sixteen years old, though, and due for a replacement due to age alone. The furnace was about 80% efficient and had dangerous Plexvent piping. The a/c was about 8 SEER and wasn't properly fused (the contacts had welded themselves "on" a couple years ago, which was a fun problem to troubleshoot).
Much like our kitchen, things worked but required more and more maintenance. I had replaced valves on the hot water tank. A blower on the furnace. Knife switch on the a/c. At a minimum I was looking at a couple thousand to reconfigure the furnace vent with a pull-fan in b-pipe, add a new drop in filter box, replace our forced-vent water heater, rewire and re-insulate the air conditioner lines. So we decided to just replace it all with a ten-year warranty.
In essence, we will recoup our costs over the next decade (except for the water heater) and not have to worry as much about breakdowns and repairs. Seemed like the right decision, but it puts off our flooring replacement another year...and I really want to replace our flooring throughout the house.