Even with skins, trophies, and stages, there wouldn't be enough to really get you excited to keep playing or do adventure mode battles. As long as they're not like play 20 hours or 200 battles to unlock like mewtwo used to be, things should be good. You should have to be able to earn it, not just grind for it.
And it makes sense that Luigi would finally be available from the start now because he's such an old, staple character that while functional to play, he isn't spectacular or exciting even compared to characters you've never had to unlock. Oh, I just unlocked Luigi? Not exactly excited. So yeah, available from the start was to be expected for him.