HL:A showed what a AAA FPS is in VR.
I wouldn't call it a killer app simply because these days, FPS is all about multiplayer, and a FPS multiplayer in VR is just...meh? It doesn't really work.
Really, any sort of action-packed combat is just not going to work in VR. Jumping behind cover works fine outside VR because the medium acknowledges the limitation. But in VR, trying to jump behind cover doesn't work when you have to worry about being next to the edge of your play space. VR tries to immerse you, but that immersion is limited when you can't touch your surroundings. Being crouched behind cover and peeking over it to shoot feels unnatural and immersion-breaking when you can't lean against the wall you're hiding behind.
Even as a VR supporter and owner of a Valve Index (And the HTC Vive before that), I don't know what the killer app for VR would be. To be honest, I'm not sure I'd even EXPECT there to be a true single "killer app" for VR, but rather, a collection of apps that work incredibly well in VR, or don't work without VR. Beat Saber comes up a lot, and it's fun, but I wouldn't call it the "killer app". It's just an example of something that only works in VR.