I mean it was quite clear what was going to happen:
Being Noctis' rival already gave her a prominent role in the story even if the trailers only focused on a few scenes. Its also worth noting, that she was more of an unfortunate enemy, opposing under forced circumstances. We know that role had been completely changed hence introducing Luna. While there are some similarities, the biggest difference is the aspect of Stella that was the most advertised and renown: her rivalry. The juxtaposition was far more captivating than what we have seen between Luna and Noctis.
Your second gif btw isn't Stella, it was confirmed to be a blonde hooded man probably the same one from the E3 2013 trailer talking about how everything is ever changing.
I think people don't understand Stellas importance/downplay her and really tend to undermine the fact at the time, we knew that only her and Noctis could see the light therefore were the only two who we knew had these special abilities in the first place.
I 100% believe that they were fated rivals because of the political positions they were in, but before Stella wasn't a Princess/Oracle, she was just a stranger to the prince, but was against Lucis and I think a lot here like that idea.
Tabata also downplays her and actually says that instead Luna was strong and Stella was weak, but in what sense?
He probably means that Luna is stronger than Stella politically, not that shes a skilled fighter because I do not get the vibe at all that Luna knows how to fight.
That picture at the end of episode Duscae itself just shows that she doesn't know how to defend her self as she manages to escape Lucis and she even has Gentiana as a body guard of sorts.
One thing I can really think of though is, it is confirmed that basically all the other nations at this point are united under the Niflhiem banner. Luna is working with Niflhiem, everyone tells Noctis to go to Altissia for the wedding, but Luna isn't there shes over at Lucis during the movie and Noctis is basically lied to by both his Dad and Niflheim and Luna basically agrees on for what purpose though? If I were Noctis I'd be pretty pissed and feel like I got played.
We can't overrule the chances that, Noctis somehow turning on Luna isn't a possibility they've only shown early parts of the game and it is a 50 hour long journey.