Since I'm responsible for both the answer and comment you point to, I don't have much more to say, although there is one capital element I feel I omitted: motors.
Track parts being just plastic, they wouldn't actually be that much of an issue to produce again. Maybe LEGO would need to make or even redesign some new molds, but if they felt there was a demand for it, that's probably not going to be the biggest issue.
However, motors and articulated bogies may be more of a problem, as they rely on non-plastic parts which may even be outsourced. Of course, this is by no means authoritative, but LEGO would have to come up with new motors, and presumably have them fit the newer PF system.
This is not impossible, and considering the PF battery box is only higher than the old 9V one, that wouldn't be much of an issue either.
Someone asked a LEGO representative at an event whether monorail would come back, and his answer was a quite clear "no plans to do so at the moment", citing difference in perception about it between adults and child.s Basically, only some adult fans want monorail, kids don't, so it would make little commercial sense to produce monorail again right now.
So technically, there would be some high costs, but no real impossibility. As far as I know, the old system was quite stable as well, so that's not an issue either. But what may be stopping LEGO is whether they feel it would sell or not - they aren't in the toy business to make losses after all. So presumably their first monorail sets didn't sell too well, or not enough regarding to their cost.
But in all honesty, we may probably never really know. LEGO employees will basically always answer "monorail is not coming back", but even they have only a limited vison of the future (and they can't tell about it anyway).
You may want to support some monorail projects on CUUSOO - who knows.