I've always been an avid amiibo supporter. I have 8 or so of em and have the Splatoon 3 pack pre-ordered. But I really can't defend the challenge mode being locked behind amiibo...
With Mario Party, lots of people like to say "1/3 of the game is locked" which is technically true, but that mode never would have existed if amiibo didn't exist. It's a special mode they designed for amiibo. So it's excusable.
With Splatoon, this mission mode seems pretty standard.. I'm sure it would have been included if amiibo didn't exist. But they're locking 60+ levels behind $35.
I understand you are paying for the figure. I love the figures. That's the main reason I buy amiibo. But for people who just want the full game and don't want what some would argue as "kid's toys", that totally sucks. They should offer the stuff as a download for people who don't want amiibo.
I love amiibo, but locking away a mode that should be in the full game just isn't okay. It's just as bad as any other company having day-one, on-disc DLC.