I dont think they will ever get rid of the monorails.............To me, it would be like getting rid of Cinderellas Castle or Space Mountain.........just not possibleYour correct it isn't a 1991 Yogo it is a large framed machine that is made to last a lot longer than the numbers I have heard here. The original trains were replaced because they didn't have the power to put enough cars on to let them pull the upgrades easily or bring enough people. I have been going to WDW since 1983 and since that time people that don't understand how machinery works have always figured if they witnessed a single time when the trains were down, then they went immediately to either they need new trains or they don't maintain them. Both are BS. Those machines will continue to operate at the place as long as broken parts can be replaced and the the cabins aren't falling off the chassis. I believe in luck to some degree but I have gone there 48 times and not once were the trains down due to mechanical problems, and that is also why they have extra trains. They don't run all of them at the same time. And if one is to break down it is replaced with another probably while you were riding PotC for the second time.
As long as they are structurally sound they can run forever and not have anymore mechanical problems than they had when they were first put on the rails. I'd be willing to bet that every single monorail train in the WDW fleet has been completely rebuilt at least 3 times in their lifetime. Perhaps one piece at a time or a complete drivetrain rebuild.
Personally I'd rather see them replace the rails. Those concrete spans have been in that Florida sun and handling tons of weight countless times a day. I'm more concerned about the stresses they have to deal with than the trains themselves. But, when they decide or and inspector decides they are no longer safe, that is the absolute day that monorails will be eliminated and gondolas will be the primary thing. Cables are easily replaced not so much those concrete rails.