I don't think there should be some deep storylines to Mario games, because that'll make the amount of Mario games limited.
But look at the Zelda Series, Nintendo has managed to generate more Links and Zeldas than a top selling computer company (hopefully the series gets to the future; we can see no Master Sword!).
If you look at Metroid, you can cleary see that all the games' timeline makes sense, but the series could end quickly.
What I'm trying to prove here is that Mario games should be never ending, even if it makes us bankrupt.