There’s another angle the author didn’t touch on. Nintendo is gating this content not only behind the purchase of amiibo, but they’re also making it inaccessible to anyone who has a pre amiibo 3DS model. So for them this ends up being not a $15 price for content, but actually closer to $200.
I’m not personally bothered, because Metroid for me isn’t a game I play for an exacting mechanical challenge. These games normally don’t have hard modes and I’m fine with not having a hard mode in this one either. But for anyone who does feel like this stuff is important, they’re really kinda out of luck if they got on the 3DS train early.