That will work as a stop-gap, sure, but having the app projected in 2D inside a VR headset kinda defeats the point. Vision Pro will only be successful if its apps can be immersive.
OTOH, the visual aspect is not the most important for Spotify (as long as the music plays) or Netflix (as long as you can play the video). None of these 3 services really need to be that immersive. On iPhones and iPads their website behaves well enough. I am sure the experience will be good enough for videos being played from a website.
The app isn’t just about playing music in the background but also browsing catalogs, curating playlists, search, discovery, playback controls, casting and more. All the current UX is designed with touch in mind, so who knows what the actual experience on the Vision Pro will be.
I've done a bunch of web browsing on the Quest with hand-tracking. All of the important touch gestures have an intuitive equivalent.