I have made it well known that I am not a current fan of much of anything that Disney does at the moment, but I do know mechanical things. Because of that I feel safe in saying that they fact that these things, in four different parks are still operating after up to 50+ years of operation tells me the whatever is possible to be a candidate for preventative maintenance is being done and being done very well. Old parts, be they metal or human cells, eventually wear out and it is not sound thinking to attempt to fix things that aren't broken. It's like getting a heart transplant when it the original is still working well, just in case it decides to give out. It just isn't done that way.
The thing the jumps to mind most often along those lines is the monorail trains. The current trains have been operating there all day, everyday for some 30 years. Does anyone honestly believe that those trains that have to have millions of miles on them would still be as dependable as they are if preventative maintenance had not been done regularly? Whether or not, they should have replaced the fleet by now, is a completely different question, but to think that they aren't being taken care of is without any sensible reasoning. And the same goes for many of the attractions especially the newer ones that are highly technical and we all know how many times we have had a computer crash at home and we don't put anywhere near the pressure on those items (computers, tablets, or phones) then any attraction at WDW dependably deals with day after day. Age wears out all of us and there are some things, including machines, that we as humans cannot forecast or prevent, it is just part of aging and life itself.