The movie is nonsensical because of the paradox of time travel
Since the person sent from the future to the past changed that past, that person changed the future as well, at the same time negating her own existence and the existence of the people that we saw manning the time portal, as well as the portal itself. So she wouldn't have been able to be sent to the past because she didn't exist, nor does the portal.
That's why time travel is a paradox in itself.
You can drill even deeper: because of the changed past, the civilization of the future that we saw, doesn't exist, nor does the portal which would have sent the surviving people into the future. The only way these people would proceed towards the future is if they simply let time take care of it.
However, there is no people to send or to proceed towards the future, since their existence is negated by the paradox.
It's just simpler to say that time travel (towards the past) is impossible. This negates all those time-travel flicks out there, I am sorry. Yes, they are all nonsensical.
The only time-travel movie that tires to make some sense is Primer.