I really don't know if I would be surprised if in another 20 years there is no monorail in WDW.
Eventually the Mark VI's will need to either be completely replaced or go through a serious overhaul. I'm not sure what the per train cost would be now to completely replace the fleet, but I believe the Mark VI was somewhere around 6 million per train. That was over 20 years ago. I just don't know if i see TDO paying 72 million for a new fleet in this day and age, let alone however much it would cost now.
Being a HUGE monorail fan, I hope I'm wrong, but I have a feeling that when the time comes, if WDW can get away with just using buses and boats exclusively and if it would be cheaper than buying a new monorail fleet, they'll just tear it down. All they would need is more buses and boats to make up for the lost monorail capacity. Then again, I'm sure Disney loves the fact that they can charge more for hotels on the monorail line, but I'm sure they'd still be able to justify those prices with just boat service and proximity to MK alone.
Also, those concrete beams aren't going to last forever. If there is ever a situation where they would need to be replaced, I doubt they would want to spend that money to replace or fix the beams if they can get away with using another, cheaper, mode of transportation.
I really never thought eliminating the monorail at WDW was a possibly, but with the way the company operates these days, I just don't know where they draw the line. Like I said, I hope I'm just being crazy and overreacting and that I'm way wrong on this one. Please let that be the case....haha.
edwardtc, what are your thoughts? You seem to have a pretty good handle on stuff like this, especially with regard to concrete, more specifically the lifespan of something like a monorail beam.