So it's actually far from trivial to draw a line.
In at least two european countries that I know of (but probably in all of them) cars need to pass periodic technical inspection to be allowed on the road. Breaks are tested, among other things.
I understand that GP point was about home-made brakes (like the software counterpart), but software on a smartphone is not (yet) deadly for others if it doesn't work as expected.
Cargo van -> Camper van conversions go through this all the time - you add/remove seats, add a lot of weight in the form of beds, water tanks, etc. add/remove windows, put solar panels on the roof... After those changes you have to take it down to the vehicle inspection, and they tell you whether or not your changes have been deemed acceptable to drive on public roads.
Funny you mention the brakes, because a friend of mine told me just days ago that he used to change his own brakes consumables (pads) until the new car, which "throws an error" if you replace the part - you have to go to an official service office for the computer configuration.
Now, do not forget that the need for the intervention of third parties lowers the car reliability ("far away", "too expensive", "device too old", "operation failure", "inexperienced operator" etc.).
This should show that your argument has difficult sides. Of course you should be able to act on your critical possessions. It should be within a good framework, but it should be fully, practically possible.
That is a nonsensical argument.
"You shouldn't be able to put anyone else in danger" - agreed.
"You shouldn't be able to modify your car" - wtf does that have to do with danger?
"Modifying brakes (not breaks)" is not the same thing as "Putting people in danger". Sometimes we modify them to have better braking than the standard.
What countries actually do is test the end-result, i.e. Does the car conform to the legally mandated required braking performance?
Rather than campaign to stop people from owning property anymore, maybe just enforce the existing laws (which, as far as I know, are enforced already anyway).
This campaign to divide people into an owning class and a servile class is pretty damn repugnant, and "Because someone can be harmed if we allow people to own things" is just the new "But think of the children" nonsense.
Say you put aftermarket brakes on your car and they fail, causing an accident that harms someone else. The person who changed their brakes should be held liable legally, its as simple as that. Owners that choose to change their car and do a piss poor job of it are held accountable for their actions and others considering similar modifications can choose to learn the lesson.
Yes that means people could be harmed in the process, but regulations themselves harm people too. There's no way around the fact that one way or another people may get harmed during their lifetime. In the long run regulations just guarantee that, should the wrong people take power, the regulations and authority that originally allowed regulations will be abused.
I'm actually surprised I haven't seen more push back on government authority given everything Trump is either doing or claiming he will do. The president should be largely an anemic office acting more as a figurehead than anything else. We've given them the power to effectively legislate with no oversight, that why he may be able to do so much harm.
But even for cars, it's quite clear that a modify-test cycle there is on the order of months/years (also, has a money burden that probably the owner has to pay). But this would 100% fail to scale to IT - like should I go to the government on each commit? Do I get a signature from them for releases?
The problem is any feasible example you come up with are already regulated, for the same reason you came up with it - there's danger to others!
Where I am, gas pipes, even inside your own house, can only be legally installed and maintained by a certified technician. You also have to get an annual clearance certificate done.
Just about everything dangerous is already regulated; further restrictions "just in case" are not warranted.