Storage space is limited. There's a black box for accidents that keeps a rolling window of data. That's not the dcm. Outside of that, how much telemetry can you store? What's the retention when there's no cellular connection? And importantly, where is it stored? My guess is that the dcm, having a battery back up and a cellular connection, is also the telemetry store. No evidence other than it's the cheapest and most reliable way to do it.

At least for Subaru, the dcm also connects to all antenna so removing it disconnects gps antenna. For other cars, I'd still expect removing the dcm to be good enough for 95% of people given the current expectation from car companies that no one would want to remove the dcm.

Good point, but in practice I think the only way onboard data could be exfiltrated is by a dealer while the car is being serviced. If you DIY or hire an independent mechanic, this seems unlikely.

That also means it isn't passed to your phone via android auto / carplay. Phone GPS is much worse than car GPS for road navigation. It's basically unusable.