The movie itself has a lot of fun with the concept of time travel and lends itself a lot of liberties with the rules it makes for itself.
Even though the movie is action-packed and filled with quips by Ryan Reynolds, it still has a sad ending if one decides to think about it more than what the movie shows.
The movie ended with the scene of Adam meeting with Laura and how their chemistry instantly kicks off their romance once again, showing that the echoes of their past versions transferred to the current ones without a hiccup.
However, let's take a deeper dive into what the ending actually gives the characters off-screen.
Louis Reed died for nothing
The entire movie happens because of Louis Reed, the father of Adam. His invention of time travel gave birth to the reality of Adam's future self travelling back in time alongside his younger version.
Here's where it gets tricky to understand, according to the movie's own logic, Louis already changed the entire timeline of upcoming events by the ending of the movie as he destroyed the drive that had the data about how to make time travel possible.
Therefore, he has already changed the timeline in a major way, however, he still chooses to not know how or when he dies as he thinks it would mess with the timeline.
The fact is, it's already messed up at this point. One cannot fix a glass that's already shattered by avoiding another crack.
Big Adam might have easily manipulated Laura to fall in love with him
Adam mentions that he did not remember his interactions with his own version with his future self.
This can lead to two theories: One, that time travel does not lend the user to be able to retain the memories of the travel itself. Two, the future version of Adam that we see in the movie is the first one to start the loop of time travel and therefore does not remember any moments of time travel in his younger days.
If we go by the logic of the latter, it means that Louis could have easily hinted at a lot of the things his future self will do, including meeting the love of his life.
Making the accident of bumping into her in his class much more forced interaction than one might have thought.
This is especially hinted at with his line at the end when he talks to Laura she says "I'm lost". in regards to how she ended up on the wrong campus to which he responds with "Not anymore, I found you". Which makes their accidental love a much more forced event.
Maya's death proves everything is still connected
The theory of time travel which says that jumping into completely separate timeline is how one can jump in time is also completely debunked when Maya Sorian accidentally kills her younger self and in turn erases herself from existence.
This event clearly shows that the past, present and future are definitely connected within the same timeline.
Therefore, the fact also carries onto the the Adam's (younger and older) who get erased essentially died instead of returning to their own timeline after their entire timeline gets altered with the decision of removing time travel entirely.
Adam got a good father at the cost of his life
The version of Adam that exists in the past where his father is still alive is the most lucky version of Adam. As he grew up with a person who decided to be a better father than a better scientist.
However, this came at the cost of his father letting go of his obsession with science and accepted death knowing it will come before he could see his son grow up to be an adult.
Louis will never get to meet the two versions of his son that made him a better person ever again
As the timeline corrects itself, it erases any and all of Adam's existence, down to their atoms.
With no access to time travel, Louis indirectly is responsible for the death of two version of his own son that helped him grow better as a father and made him realize that science is important but family is another issue altogether.