I'm sure PanAms were fine but these are the Olympics, the scale and the importance are totally different. The other stadium looks fine but I think I would be disappointed if I made the Olympics then I couldn't compete in the Olympic stadium.
No plans are official but during the bid process Rio16 organizers said that the marathon would not start or finish in the Olympic stadium either. Instead they plan on starting and finishing in the Sambodromo.
It makes sense to do it this way, Rio certainly doesn't need two stadiums with tracks. They already had the alternative one from 2007 so they would have to construct a track at Maracana and then probably take it out after, which doesn't make sense. They obviously plan on selling more tickets to soccer matches than athletics.
I just get the feeling that this move is pushing athletics to the side in the one event every four years where it actually matters to normal people. If the games are more financially successful because they sell more tickets for soccer games then that will be the new model for the Olympics - the little World Cup.