Maybe. I think the realistic problem with this strategy is that if you take an existing human and help him in some obvious way, then it's easy to see and measure the good you're doing. It sounds pretty hard to figure out how effectively or reliably you can encourage people to have happy productive children. In your thought experiment, you kill the hermit with 100% certainty, but creating a longer, happier life that didn't detract from others' was a complicated conjunction of things that worked out well.