I've long had this issue with amiibo -- it's physical DLC. Plain and simple.
You're tying in-game content, regardless of complexity or importance, to a physical item. Typically, DLC will run anywhere between a dollar to 15, maybe 20 for extremely substantial content (xpacs and other new story content). But these prices rarely change, and when they do, it's typically a drop in price. But Nintendo's tying DLC to physical objects that are constrained by supply and demand, and therefore are subject to upward price momentum. amiibo can be discontinued or underproduced, sending their value skyward, and with no alternative way of offering the in-game content, the DLC becomes increasingly difficult, if not impossible, to obtain.
The obvious solution would be to offer amiibo-locked DLC as a separate download with a lower pricepoint, but that reduces the appeal of amiibos, so I'm not sure Nintendo would do that. TBH, I'm surprised more noise hasn't been made about this. Especially with all the pushback against pre-planned DLC, season passes, full-price games with microtransactions, etc. This just seems to be the worst offender, yet it's received comparatively little attention. The primary rationalization is that "the content isn't important anyway." Sure, that may be true -- albeit subjective -- but what happens once something substantial IS offered behind an amiibo? Even beyond that, though, is the sheer principle of it: DLC should not be constrained by real-world supply.