At some point things do become public domain, unless of course you've got enough money such as Disney who intend to hold onto Mickey Mouse for ever for example.
Even more stupid are things like the rights to
Happy Birthday being held and you'd better pay royalties if you wish to sing it for a profit!
Back to cars. Pure race cars such (as the 917) seem to be treated differently and others such as the 7, '40 and Cobra seem to have got a confused enough history to make it impossible for the barn door to be closed on those designs.
If you search the web you'll find people doing really horrible things to crappy old cars to make them look like Ferrari's or whatever I can't workout why it'd be in Ferrari's interest to let this go ahead. Ferrari & Lambo don't want to sell cheap cars, if you can 't afford the car you're supposed to buy the T shirt or hat or whatever other crap they condescend to make for the average person, anyone been in a Ferrari store for example?
On the other hand looking at GT-40's I think their actually doing ford favor by keeping the memory alive of it being a performance brand.
The 10 year thing is a bit of a joke, how long for example is Fran allowed to make SLC's before someone can splash it and make a copy?
The lack of clarity in the situation is one of the things that tinges the whole kit-car industry, if there was a clear set of rules on the subject then everyone would be better off for it.