It's a question of ownership. If I see fit to modify something I own, as has always been done, why am I not free to utilise a particular piece of my purchase?

If I'm not free to do so - then do I actually still own my own product? Or is it now a lease?

Why should the era of rebuilding your car simply end? There are already safety regs you need to comply with. The average hotrod modifies a considerable proportion of the car. Reusing the majority of the structure in ways the manufacturer did not intend.

I think you're talking about something related but not discussed here, which would be making car modifications illegal. Let's say BMW make heated seats an optional extra, which it already is. What if they also install them in every car because it's cheaper to have one build, and subsequently are able to sell them more competitively - none of that would require making it illegal for you to get the seats working yourself.

Not illegal, in so far as I don't believe it's been tested in court - but it is an explicit breach of the giant terms and conditions contract that they require you to sign, when purchasing the vehicle. There is an attempt to prevent you from doing so.