He cloned a business model known as drop catching that has been around for a long time with other TLDs most notably for .com domains. If you look at how those markets evolved, there is an initial burst of profitability and then intense competition which leads to commodification and margin compression, which is what I predict will happen here as well. The smartest operators made most of their money by selling off the premium domains they were able to drop catch early, which seems to be what he is doing also (& kudos to him for what seems to be excellent tactical execution) - but the drop catching service itself has steadily less value over time and of course, even in the absence of competition from peers, is wholly dependent on the .io registrar's particular rules regarding drop catching which can change at any time.