The fact that you have the option to purchase only a few characters if you so wish is a good thing. For people who don't have a lot of money, and really want to play KI the fact that they don't have to spend $60 all at once to enjoy the game is an amaing choice to have. There is, however, nothing stopping anyone from buying the full game.
If you're concerned about digital only, I'm fairly certain it was already stated that a retail release would be happening as well, just not right away, so you can relax.
I am not a fan of the game so i couldnt care less wich platform is getting it, but i recognise a game that can be interesting for those that like it. I will just let you to wonder why games like MK komplete edition, SF4 Arcade edition, Ultra street fighter 4 come out on retail. You have a long ponder on that and try to picture how that is important to tournament organizers that need to have multiple machines running with the same version of the game.