Regardless of how much Luminara did in the fiction, she is a large part of the game.
Most of us not only used but relied on Luminara at some point, probably as the healer on our first teams. Almost all of us have her at seven stars with more gear on her than seems practical in retrospect. She even used to be meta.
What's more, she's highly accessible early game. Giving Luminara a rework is one of the biggest legs up they can give to as many players as possible in one fell swoop.
What's more, the theme after ST stuff seems like it's gonna be Galactic Republic Jedi. She's a Galactic Republic Jedi. If she becomes good, that's just easy material for a new team right there.