This is not legal advice, and you should consult an attorney if you believe that an OEM may come after you for tampering with your own vehicle.
That said, I believe that there is very limited legal recourse for a vehicle OEM who wants to prevent an end user from changing the data collection and upload features of their vehicles. The worst thing would be potentially voiding the warranty, but for 10-15 year old used vehicles, that's not an issue.
I work as an in-house counsel at a vehicle OEM (likely not one you've heard of). Our end customers are generally large companies, so it may be different for personal passenger vehicles, but we actively plan for the contingencies where our end users switch out their telematics module, or request to have the telematics data stream be directed to another service provider. Further, in our market, the idea that the customer wouldn't want telematics at all is not really likely, but we also plan for that possibility.
So, I don't believe the used market isn't as big of a deal as you may think. The real concern for personal vehicles is losing essential warranty or service options for new vehicles by turning off the data collection.