If there's no law to prevent it, "traditional" cars will also soon come with unskippable ads. At least, that's the way how "traditional" TV manufacturers went. And if there is a law against it, Google or VW isn't a meaningful distinction anymore.
We’re down to a duopoly in the smartphone market. For most of us without a desktop/laptop (the bulk of global internet users), our digital lives are essentially controlled by Apple and Google.
Why would this be any different with SDCs? The tech might be getting easier, but current market forces make it far easier for the big players to monopolize the market.
There are a lot of competing car makers. In theory, they should select from multiple competing offerings.
Then again, they haven't always done well with that in supplier selection. You might be right, they might end up largely tying themselves to a few suppliers.
OTOH, the network effects shouldn't be as strong. It isn't hard to imagine FOSS options like OpenPilot continuing to exist.