What would be peoples motivation to buy it for existing computer?
* "It's sorta legal, it's at least some kind of a licence, right?" -- they're misinformed / tricked /fools
* "I want to pay something, but retail costs too much" -- if you pay without getting anything useful in return, you're donating. Why not donate to charity?
* "I want to get Windows Updates" -- cracked copies used to get updates, and they're often bundled with updates already applied
My point is, I don't understand buying half-valid licence. Licence is either valid or you have no licence.
Because your sense of fair play says you've got something from MS so they, and not some random charity, deserve some of your money? Do you follow the same logic and donate to charity instead of tipping at restaurants?
The idea of buying a chef's knife from Victorinox for one price if it's to be used to chop vegetables and another if it's to be used to trim fat from steaks is absurd. You buy the knife at the lowest price it's offered and use it however you like.
Software licensing does include the ability to charge different prices for different usage scenarios, but violating those conditions doesn't strike most people as unethical while obtaining the software without paying seems similar to theft.
Purchased goods are purchased goods, and the idea that something is licensed with restrictions seems ridiculous to most people unless it's something upfront like a subscription that you are paying monthly fees to access.
I know plenty of people who would hate to torrent of, say, Photoshop, but also don't think it's worth $550 to them. Having a bought license that's legally of questionable origin is a compromise many non-business users would be more than willing to entertain.