If Luke was there to see it then he would be there to face Kylo and take away that moment from Rey.
Rey is the focus. Rey is who the series is about. Sticking Luke there at the TFA final battle destroys the focus. This trilogy is about Rey. Nothing should take focus and attention away from her.
She doesn't have her moment with Luke if he is on starkiller base with them..
Michael Ardnt said it best, if you had Luke in TFA it pulls focus away from Rey which breaks the movie. The movie had to remain Reys story. Rey had to be the one to face Kylo at the end of TFA as her heroic moment of accepting her destiny.
They bent over backwards to write the story to showcase the new characters and sacrificed the integrity of the old beloved characters to do so.
They've outright admitted that they authored the OT character's stories and screentime to make sure to not get in the way of new characters. That is not, not, NOT how you write a good story, particularly a story that includes beloved characters that have been fleshed out by three full movies.
Technically Obi-Wan had 3 movies leading up to Episode IV --- so did Anakin Skywalker (Vader) and Yoda. Luke, Leia and Han were the new characters. Did George Lucas write Obi Wan's and Vader's stories for Episode IV - VI strictly to make sure Luke, Leia and Han got the limelight?
NO --- Obi-Wan, Vader and Yoda got the story that they deserved in a way that was amazingly satisfying and true to their character arcs. They got the screentime that the story (a damn good story) demanded, NOT the screentime that was required to make sure that Luke, Leia and Han remained at the forefront.
The entire ST should have been different. Here are the major things that needed to be different about the approach that Abrams, Kasdan etc. took when writing the story:
- Write the story that should be told. Don't bend that story and twist the screentime and character arcs of beloved old characters just to make sure that they don't get in the way of your precious new characters.
- Don't approach the character arcs feeling that you need to do something that defies expectations, just for the sake of defying expectations.
Fans think Han and Leia are still together? We'll show them!
Fans think Luke is a great, wise Jedi Master? HAH! I know what will surprise them!!!
Fans think Snoke is Plagieus? WOW, they will never believe the true answer, that his character is so trivial and expendable that his backstory never needs explaining --- they will be so surprised!!!
Fans are interested in Rey's parents? Hmmm.....what would be the answer that no one would guess (Rian Johnson actually admits to this being his central motivation while brainstorming Rey's backstory).............her parents are nobody-no-names!!! Junk traders that abandoned her! WOW, fans will be #SOSURPRISED .