I'm not in a position to know but from reading around a lot I think the VW ID3/4 ended up having a nightmare update scenario because early hardware was buggy.
I know from working at a phone company that hardware absolutely has bugs and it's only reliable as long as you're changing very little. If you start having a system where you're trying to optimise everything e.g. to save power when not in use and allow lots of customer choices in how things work then you're going to choose new microcontroller hardware and software and it will have bugs that have not been ironed out over the years.
The stories I read about ID3 updates going wrong were about certain controllers not successfully acknowledging their firmware updates. So I can see a strong temptation to make it all vertical - to have control over everything so you can ensure it works before you order 200k of them. Instead of having 60 suppliers all providing their interpretation of your bus protocol, each with unique bugs you take the whole thing into a single design.
I can also see it being far more conducive to have a smaller number of bigger and more homogeneous microcontrollers each of which might not be perfect for every job but since they're all the same you only have to provide 1 software update solution and you can test the shit out of it.