It's not. If we count every Mario game as being the same thing, then yes, it is milked hard. However, as i don't see any gameplay resemblance from Paper Mario to Mario Tennis or NSMB to Mario Strikers except the fact that these games have the same characters, i can't say the franchise is milked. In fact, given the sales of the Mario series sub-franchises, i would say, absolutely, that the series isn't milked at all.
For example, let's see release dates from the Paper Mario sub-series, which is a RPG:
Paper Mario(2000)
TTYD(2004)
Super Paper Mario(2007)
Sticker Star(2012)
So, roughly, what we have here is a quite reasonable 4 year space between new releases on this particular sub-series and only 4 games made for it in 12 years of existance. Absolutely not milking. Now let's compare that number to another Mario sub-series, Mario Kart, a racing series:
Super Mario Kart (1992)
Mario Kart 64 (1996)
Mario Kart: Super Circuit (2001)
Mario Kart: Double Dash‼ (2003)
Mario Kart DS (2005)
Mario Kart Wii (2008)
Mario Kart 7 (2011)
So that gives us about 2 years between new releases, with 6 game releases, including portable ones, over almost 20 years. Not milking at all when we take in account how many little games have been released for a whole 20 years, i believe it's a reasonable time space between releases.
Now, let's get another series outside Nintendo for comparisons. What about God of War, from Sony, which is a series that's, in my view, not accused of milking by some gamers?
God of War(2005, PS2)
God of War II(2007, PS2)
God of War: Betrayal(2007, Mobile)
God of War: Chains of Olympus(2008, PSP)
God of War Collection(2009, PS3)
God of War III(2010, PS3)
God of War: Ghost of Sparta(2010, PSP)
God of War: Origins Collection(2011, PS3)
God of War Saga(2012, PS3)
God of War Ascencion(2013, PS3)
So what we have here is, roughly, less than a year space between releases in a 8 year old series, that has 10 games which have the exact same gameplay(hack'n slash). And that's not accused of milking.
PS: Not trying to criticize God of War or anything, because it's one of my favourites hack'n slash games ever. It was only mentioned for comparison purposes.
