Vudu summed it up pretty well The law of deflation/inflation is meaningless when you account for the law of dissemination. Bill Gates may be the richest man in the world(i don't know if he currently is or not), but just because of inflation doesn't mean he's going to give away all of his riches. He's still better off than all of us.
Because Mario Galaxy is one of the highest rated games of all time one could say that a large number C level game spinoffs could be approved. Thinking about actual Mario spinoff releases though, i find there to be less off the top of my head than were on gamecube. The Mario party series has ground to a halt after 8, there are two sports games each usually with a B+ rating (baseball,soccer), Mario Kart is an beloved series in its own right (also, fairly well reviewed), Super Paper Mario is a mixed review, and New Super Mario Wii is something most people liked. If anything the Mario party games were dragging down the brand, but there hasn't been a new one since 07.
as far as Zelda goes, i tend to seperate hand held and console offers completely. There have been 2 Zelda games on DS, neither were terrible, maybe mediocre, possibly great depending on opinion. Twilight Princess is either appreciated for its fan service or just plain meat, or despised because of its all too-familiarity. Crossbow challenge is more like some sugar to go with your kool-aid. Its not meant to be a be-all end all Zelda game just something to do with your gun. If it didn't have the Zelda theme we wouldn't be talking about it, but if it didn't have a Zelda theme it wouldn't be developed at all because they were just recycling assets for a low budget bargain pack-in. Nintendo should develop some new IPs, but that i easier said than done. For Nintendo developing a new Ip is more than just putting new characters in a game type, its developing a new genre. Each game EAD makes though, they do little experiments, which is see as testing the waters.