I understand that they way the company is going these days, we can't put anything past them. However, these dire predictions about the monorail are moving into the realm of hyperbole.
First off, the Mark VI trains are transit-grade, they are designed to be refurbished, refitted, and refreshed many times before replacement, as seems to be currently happening. The 20/30 year "replacement" cycle may be valid, or much like the 120+ year-old steam locomotives (usually) circling the MK daily, they may require a large overhaul in that time frame. Let's not forget that the Mark VII monorails in DL are still built upon the Mark III frames dating from 1969. This shows that the trains, with occasional major overhauls, can last a very long time.
Secondly, the beams. Again, let's look to DL...original beams still in place since 1959. By the way, the EPCOT beam is in much better condition than DL's. (Admittedly this is anecdotal based upon the "feel" as one is riding... you can actually feel the "sag" between supports on DL's beam...never noticed that on any of WDW's.) Again, WDW's beams are significantly larger and "heavy duty." Also, even if the beam were to require replacement in the future, it is not the same expense as a new route, so long as the support columns, or perhaps even more specifically, their foundations, remain solid.
I don't believe any leader in TWDC is so stupid as to think the monorail is expendable. It is iconic and integral to the resort's image. The day that TWDC announces that they are permanently closing the monorail - or any portion of it, it would scare the hell out of shareholders and other investors. They would largely assume TWDC is about to either go out of business or shut down the resort operation, either of which is signalling desperate straights.
I guess let me bottom line it this way: the day the monorail systems dies is, at best, the beginning of the end for TWDC.