Guild wars 1 had a model based on selling the main game plus expansions and it worked very well. You can also have a cash shop (cosmetic items, character slots etc. only please!), which would become even more viable if avatars get added and you can see your ships more often, from the outside. This is also the best option for players, I reckon, noone who doesn't play constantly wants to pay monthly and free to play usually ends up as pay to win and constant in game ads that spoil the gameplay.
Also Elite Dangerous is not on places like Steam. That means they don't have to follow the Steam model, where games start out highly priced and then are given away for next to nothing in the next sale. So hopefully ED can maintain it's value over a long period and not have to sell maasive volume only in the first few weeks/months. And that model also gives Frontier a very big financial incentive to improve the game on a constant basis, because that is what will generate new full price sales and also it encourages them to release expansions that get good reviews, because that will generate further sales of the main game, plus expansions.
This model is really how I'd like to see any server based game I play run, because it is the one that maximally encourages the developers to improve the game. And games I really like are about as rare as hen's teeth, so lets hope Frontier make it work. I think the launch has been successful, but it isn't easy without seeing the volume of sales, to estimate just how succesful - maybe DB will enlighten us, at least to some extent, in a future newsletter.