Desmond couldn't be with Penny because he had to get to the island to press the button. Of course once his code entering got screwed up by Locke (and what isn't screwed up by Locke?) and he turned the reset key, well, everything got reset, didn't it? What his ultimate destiny is was not foretold by old giftshop lady person so Desmond might be free to sail into the sunset with Penny now forever more. I don't think there's even any need for him to return to the island, except in an alternate lifetime...