Every large structure/machine needs various levels of maintenance at various times. Things may pop up everyday with just a small tweak. Every now and then, a more serious bunch of maintenance may need to be done, like a new motor or other component. Then, every few years, a major bunch of maintenance is needed, which actually shuts down the thing for a while. This would include deeper and larger components, and is usually done on a rotating basis at Disney, often shutting down the attraction for a few weeks or even months. No machine can function without normal maintenance, and no machine lasts forever without an occasional major rehaul. Your car will probably last ten or 15 years with normal maintenance, but it probably won't go 20 years unless an engine or transmission gets replaced (a major rehaul).You make it sound like Disney cannot afford to keep up their own attractions.
And then, after perhaps ten years or so for a Disney level attraction that runs thousand of hours a year, a major major major rehaul needs to be done that might require Diseny to gut and rebuild the thing. When this happens, big time decisions have to be made as to whether or not to keep it the same, to do a major upgrade, or to tear it down and start over. Test Track got a major upgrade. Horizons and World of Motion got a tear down and start over. Big Thunder Bucket Railroad has essentially been kept the same. Many elements go into the decision, most of which is whether or not the existing attraction is still popular and/or could be made popular enough long-term to justify keeping it essentially the same. Universe of Energy was very cool years ago, but the novelty of a slow mover like that has passed. I believe it will be leveled and completely replaced.
Test Track, most would agree, is far better now. It was popular when they closed it down (which was shortly after Soarin' first opened), and is even more popular now. It is also a hugely expensive attraction to build and to run. It's not going anywhere.
To respond more pointedly: Yes, Disney can probably afford to keep up their own attractions. It's just that a tweak here and there or overnight won't cut it after wear and tear really sets in. At some point, a major maintenance shutdown is needed, and after many years a decision as to whether to keep the thing has to be made due to a variety of factors, including big time wear and tear AND whether the renewed attraction would still bring them in. Expedition Everest will be getting a major major major rehaul within the next couple of years. It's worth keeping. Test Track is worth keeping. Horizons and World of Motion were deemed not worth keeping. Regardless, every attraction at some point needs a major shutdown no matter how well it was maintained. My Dad and Brother-in-law were maintenance men, and they talked about such things often. I hope this helps clarify this issue.