Perhaps an easier way to handle some facets of this is to allow "transfer of product license or ownership" between accounts.
As example, if you have more than one iTunes account or another type of Mac ID, you would take what you bought and transfer the ownership of those items until you empty out the account. This doesn't solve everything but its a good start. I know when I started I used a non Mac account for iTunes and later got a Mac ID/account.
Candidly, Apple doesn't have to do it all at once. But they certainly could have made inroads. The word that comes to mind is 'pathetic' that Apple would like to be consider so advance and forward thinking and still lets these issues hang. - What an inexcusable debacle on Apple's part.