You have no idea how to use the move if your'e saying this, you do send Luma out and wait for someone to just reflect it, nor should you use it close enough to them for that. You need to use it and then move with Luma, grab the opponent if they have a reflector and toss them into it. You need to look at Rosalina's moves a bit more.
Also launch star attack is obvious, it makes her like Zero Suit Samus, killing really high up with her up B.
Launch Star Attack isn't even comparable to Boost Kick. Here's some data I gathered from training mode:
ZSS Boost Kick:
16% damage
Kills Mario from FH height at approx. 70% (off top)
Kills Mario from ground at approx. 85% (off side from starting position on FD) or 95% (off side from center of FD)
Rosalina Launch Star Attack:
12% damage
Kills Mario from FH height at approx. 140% (off top)
Kills Mario from ground at approx. 175% (off top)
It's an inferior attack no matter how you slice it, and it cuts Rosalina's recovery distance by an absurd degree. I would personally never consider taking it.
But even so, Rosalina already has a 2331 set. That's Luma Warp/Shooting Star Bit/Launch Star Attack/Gravitational Pull. That really only leaves Power Luma Shot as a point of contention and while I'll concede it's relatively unexplored as a move, I also have yet to see anything demonstrating its usefulness in general, especially compared to Luma Warp and in the face of Floaty Star Bit which fills basically the same role, with admittedly less damage, but without tying up Luma in the process until the move completes. (And Luma is NOT invincible during Power Luma Shot unless you fully charge it, anyone can poke it and end the move early.) Launch Star Attack's niche is fairly obvious as "Rosalina's only recovery with a hitbox," mediocre as I feel it is, but Power Luma Shot has "slow moving Luma" which is basically the exact opposite of what Rosalina wants at this point in the meta.