Agreed but there is a price difference to take into account to. Likely the key resellers sold the key for much much cheaper than Steam, that's the whole appeal, isn't it? So maybe some people would have still bought it on Steam but others may have been put off by the price. Now if the game on Steam went on discount later for the same price as the key reseller sold it for, you could definitely argue for lost profit since no one would buy the key twice.