Alternate timeline ending - implications
At the end, David is returned to a point in time right after his original abduction. However, this is the 3-day-older David that has been through the alternate timeline. The original David and Max are on a ship heading to Phalon.
8 years later, Max will calculate that it's too risky to send David back to 1978. He then will dump him in 1986. David in this timeline will find his older self and family. If he stays in 1986, it would mean he was never sent back to his original timeline in the first place. That kind of contradiction would force the timeline right back to when he was missing, making all of these events continue unresolved on an infinite loop.
So the original David still needs to find Max at NASA in this new timeline so he can get sent back. It will take at least one more cycle(likely more) to bridge the timelines so that both Davids have the same experiences, synchronizing the timeline. Otherwise there can be no future beyond the looping events.
TLDR: time travel is complex.