See my EDITRA. Stella != Luna. Stella has been removed and Luna has been added. If it were not for the fact they are both girls and one popped out while the other popped in, I don't think anyone would be saying they are the same character. But you seem to be more annoyed at people getting upset over Stella's departure than people are about Stella's departure based on this thread. xD But yeah, read the words Tabata stated - they are not the same character.

Also note that I've repeatedly stated that I think once we see more of Luna we'll forget about Stella, but at this moment in time we have indeed lost one of the very few non-playable characters that we had any kind of personality given to. So let's just wait for more information on Luna so we can move on!

EDIT: Alternative wording of the pre-mentioned quote, according to another reddit thing (I can't watch the whole video at work)...
We decided not to include Stella in FF XV. Instead, we have a new heroine named Luna, who has a different role within the story.