But why, some say, the moon? Why choose this as our goal? And they may well ask why climb the highest mountain? Why, 35 years ago, fly the Atlantic? Why does Rice play Texas?
from http://www.historyplace.com/speeches/jfk-space.htm In case you are wondering, Rice is a small university in Houston, TX, and gets regularly squashed by the University of Texas in football, and was the site of that speech.
Your example fails in every way possible. First, we have already established that a previous install exists is a tautology. Therefore, Apple does not need to check for a previous install, and does not need the draconian checks that Windows has on upgrade versions, and will happily install itself on a clean hard drive. This provides a much better user experience, without having to prove that the older version exists, either with license keys or checking for an old installation. Secondly, even if it required a previous installation, you could restore the original version from the recovery disk, and then install the new version. So, in no possible way could this be construed to be your scenario.
Consider this example: I have a Mac with OS 10.4. I buy a new hard drive, put it in, and install from my shiny new OS 10.5 DVD. Knowing that this is a mac, and therefore a previous version of OS X must exist, 10.5 happily installs itself. I also have a PC with windows XP. I buy a new hard drive, and install vista from my shiny upgrade DVD. Before Vista will install, I must prove that I have a valid XP license. I must either put in another license key, or let it install over the old XP system. Now, which system sounds more user friendly to you?
You knew the job was dangerous when you took it, Fred. -- Superchicken