Even if they manage to get big players like Spotify to develop apps for them, a lot of people - at least on the Android side - have smaller, niche apps for audiobooks and podcasts that would never bother to port their apps to GM services. Heck, even Apple Music and YouTube Music wouldn't bother. I smell an upcoming BOGO deal on their overstocked dealerships, just before they get another bailout check.
Also, what will that look like 10 years from now when "popular app of 2024" is dead and gone, and new thing has no reason to build an app for a 10 year old car. Theres a reason why people like these mirroring systems to much, let the device that is likely to be replaced every 2 or so years and get regular updates handle the software.
I said it above, but it's worth repeating that they're doing the exact same thing that TV manufacturers do with their shitty stores filled with outdated apps versus regularly updated third-party solutions like Roku and Firestick. Shit like this means developers have to spread themselves thin to maintain multiple different versions rather than focusing on one, which usually results in the niche versions getting neglected or quickly slapped together.