We don’t disagree. You can do anything you want to the car yes but the IP does matter. The manufacturer can for example lock / hardcode the ECU / software to the car and if you sell that component it’ll be inoperable in another car. This is my point - not your ability to modify the car which you can do anywhere.This is not true in the US. First Sale Doctrine is long established, essentially bedrock law here. Once you own a physical asset like a car, you can do absolutely anything to it that you want and the IP inherent in it doesn't matter at all. That includes software, as long as you aren't copying and reselling it.
The restrictions on modification to ECU code etc are 100% related to emissions controls and enforced by the EPA, nothing to do with IP protection. And it remains to be seen whether emissions enforcement of that type will stand now that the US Supreme Court ruled against the EPA regarding their rulemaking authority in the WV vs EPA case last month.
There's a long tradition of extreme vehicle modification being legal in the US. You can chop the roof off, change the number of seats, exit the exhaust to the side, double or triple the horsepower, and convert it to a three wheeler if you want. The only meaningful restrictions are applied at the state level based on their safety inspection policies.