don't you have to keep your hand on the throttle to move the train? if so, a heart attack or falling asleep aren't likely i'd imagine. the train should have stopped in plenty of time with all of the visual indicators if something did incapacitate the driver. when monorails stop due to traffic, you can't even really see the train in front. and in the case of the TTC, all the more reason to wonder given how well it'd be lit.
and i'd almost bet if the driver was that tired, that some protocol would have been breached. but again, i doubt this the case.
i also doubt this will end riding in the front. people dying after riding attractions or even just getting injured on attractions closes the attraction for a short while, but not permanently. this is reportedly the first accident the monorail system has ever had at WDW and pending the investigation results, the one incident shouldn't cast dispersions on the performance of the hundreds of daily successful runs for decades.