Lets be honest, the average punter is unlikely ever to be able to copy a part or mechanism even if they have one in their hand – not lack of intelligence, but the lack of experience.
Even if they could, the time we would be willing to put to do so in is enough to make most people buy one instead. Even most folks here get the garage to fix their car…
An example – the technology for printing your own magazines at home has been available for a couple of decades. With decent laser printing you can have something comparable to the commercially printed product. Yet the proportion of MEW readers who have a digital sub and print their magazines is tiny.
3D printed items will always take a finite time to produce and need assembly if nothing else. In the real world printers will jam, run out of materials or glitch now and again. Just because you will be able to print your own stuff, it's likely most people will prefer just to have the whole thing and get 'instant gratification'.
Even when significant numbers of people are getting designs to print off the web, the way it will go will be licensing.
Here's ways it could work:
Designs for gadgets online with same day despatch of non-printed parts.
Buy a machine – with ownership comes lifetime access to STL files for all the printable parts.
Buy a radio – you choose the spec for the electronics, and get emailed the files to build the case in a style you chose (and possibly customised) on line. If you wnat a different look in future, just go back and get a new case design and swap the internals.
Order premium parts in glass filled nyon or similar to 'upgrade' your home prints.
Designs given away as 'free gifts' i.e. as a marketing tool. Perhaps in the future we will be doing this for MEW!
I'm sure there are 1001 other ways of getting money from 3D printing.
Neil