I don't think it's that they haven't realised the possibility. It's just that it doesn't come with any promises, and they could decide to close the door on it at any time - as soon as they decide there are too few users of legacy versions to be worth pandering to any more, OR they decide they're losing too much money by honouring ancient keys, and need to "encourage" purchase of up-to-date versions.
It doesn't much matter whether you're buying the actual software or not. I'm sure they have a system for keeping track of which keys were issued with which versions, and it would be a simple matter to decide that six- or seven-year old keys aren't going to be accepted any more.
In response to your other post, I'm sure stolen keys are not an issue if you go to a reputable retailer like Tesco (but a lot of people probably said that about horsemeat!
) If you go to eBay, who knows?