So much of the film is "wouldn't it be funny/exciting if" and then forgetting to set that up beforehand. So it's a Hollywood "stuff just happens" plot.
Also a major part of this film is that there is some sort of anomaly that allows for time travel and breaks causality, possibly also conservation of mass. Other films would have a field day experimenting with this - trying to deliberately mess with time. Imagine, for example, a rich man trying to live forever by periodically displacing his younger self. Or someone else trying to undo a horrible personal mistake. Or a lunatic trying to radically alter the future. Or a sane person trying to escape the present and venture into the future (isn't that a mood?) Just that part would be a film in itself, like it is in Contact. In this film, it's basically just a plot device to both introduce the conflict and resolve it. Also, to give it an ending that isn't horrible. Not quite a deus ex machina, more like a save scum ability. You know the end of Deadpool 2? It's basically that.