The Monorail only had an overhaul a few years back didn't it (2006-7?), when they replaced all the drive systems with newer, more up to date technology. Ultimately the ride will only last for so long in its present state, but the likes of purchasing new trains would be incredibly expensive, and no doubt result in a system-wide overhaul to make them compatible. I doubt you could just take a MKIII(?) train from the EP Express, pop it on Towers' antiquated setup, and watch it work.
As far as I'm aware monorails are still a pretty expensive technology, even all these years on. The original installation supposedly set Towers back around £12M back in the 80s, so it was a major investment for them! At least with Towers' setup it's not as frequently used connection in the same was the Skyride, EPE, or WDW Monorails are. Your average guest probably spends less than 5 minutes a day aboard one of the trains, so to Towers from a financial point of view as long as they can keep the cosmetics half decent (I'm not saying the trains are perfect at the moment. There are improvements which could be made) it's not such an issue.
Maintenance to the Monorail is pretty much ongoing, as is my understanding, with trains being taken out one at a time and repaired/upgraded to extend their life. Everything has its expiry time, and those trains probably should have hit it years ago, but the techies still manage to keep them soldiering on in one form or another. There's only so much you can do to this sort of tech though, even less when the manufacturer stops supporting them, but aside from the niggles we as enthusiasts hear that the trains have, it does tend to be fairly problem free. It's not like it goes down every day and causes major headaches.