You're assuming such a system would be someone emailing you privatekey.txt

That is, of course, a system that exists, but, be creative, lots of ways to skin that cat. In apple's case, well, they could certainly sell a device to handle that job, basically a key fob like you probably use in your car (sometimes built into the physical key without your knowledge).

You could also use the remarkable amount of compute that most people carry around in their pockets to do this job with no additional device required (use your phone).

Not really a hard nut to crack overall.