There is a narrative about the monorail system when you ride from Epcot back to MK (or is it the other way?) and it talks about how old the trains are. I don't remember the exact years, but the 1971 originals were replaced twice. Once in the early 80s and again in the late 80s/early 90s. So the current trains are pretty old. They definitely have some issues, but I think it's a bit overblown, kind of like the whole "1/2 of Hollywood studios is closed so don't bother" silliness. We stay off site and MK is our favorite park, so we ride the monorail often. Yeah, it's down way more than we'd like it to be, that stinks. It needs updating/upgrading for sure. But we've never been stuck on it for any real length of time. When it's not running, our experience is that they do a nice job of using the ferry and/or buses. The ferry can carry a ton of people and there are 3 of them, so they can get a lot of people back and forth. So while I completely agree with others that I'd love to see the system updated, but I don't at all see it as a detriment, I still see it as an attraction.
What I wonder is what the main problem is. Is it the cars, the track, or both? If it's the cars, is the fix as "simple" as taking off the old and putting on new? If it's the track, does the whole thing need to be torn down? I can't even imagine how much of a construction nightmare that would be.