I think that's part of it, but definitely not the entire story. People have a lot of their life on their phone, so the tighter the integration between their phone and other products, the more appealing the experience with that product is. No matter how good the automaker system is, if it's not basically your phone with a different screen, it's going to feel like an awkward disconnect, especially when there are plenty of alternative cars that do offer that seamless experience.
I’m with vineyardmike. The problem was they sucked before CarPlay.
Now that I’ve had a taste of controlling everything my way with my apps based on my preferences in a way I’m used to, I’m not going back.
The best you could offer is perfect integration with my phone and the apps I use. But that will never happen, and at that point I might as well use CarPlay anyway.
Cars need their own system for fallback but it’s a cost center. Don’t try to make it your primary selling point, you can’t win.
If it actually works good people might not care in the medium term. Generally the problem people have with the automaker systems is that they sucked, not that they weren't CarPlay.