You're making a lot of assumptions here:
"a mode that wouldnt have existed without the amiibo": You can't say this with absolute certainty. It could have existed just fine without the amiibo as an in game unlock.
To assume the opposite is a little silly, though. This fusion mode has no history in the Metroid franchise, at all. Metroid has had a hard mode before, and this one has it too, without the amiibo. There is no history of alternate costumes. The only amiibo I can think of that MIGHT have been cut content was the Twilight Princess amiibo for BOTW.
I just don't see a good reason to assume the opposite. You don't need "absolute certainty" to assume, far and away, the most likely scenario. Maybe not in a court of law, but this isn't that.
Yeah it could have existed just fine without the amiibo, I suppose. But that's not how any of this works. These things take extra time, and extra money, all of which Nintendo gave them because they knew they would release an amiibo along with it.
"Just buy those tag things for $2. It's not your fault Nintendo doesn't know how to stock...": So your solution to a shitty practice is to tell people to participate in a secondary shitty practice of piracy because Nintendo can't even get enough product out FOR their shitty practice.
How about just admitting it's a shitty practice and getting rid of it entirely?
You are being waaaaay too altruistic. Nintendo doesn't even seem to care. Piracy, in no way, is hurting amiibo. They are selling out, and no one can get them, except scalpers, which Nintendo has no interest in curbing. I guess, purely philosophically, it's bad, but, realistically, it does zero damage or wrongdoing.
Yeah, the practice isn't that great, but it's not as one sided as you make it out to be. On one hand, it keeps valuable content locked behind a toy, on the other, it's the most valuable amiibo. For the most part, amiibo haven't given anything that good. This is one of the first ones to actually make the purchase worthwhile. I don't mind Nintendo experimenting by giving amiibo better content, overall. They do have to figure out the stock issues, though, or give some other solution. Until then, the tags are just fine, IMO. I'm a little hesitant to say, "Just release the content separately", as that kind of ruins the charm of amiibo. I support amiibo's in their current form, as a statue/DLC hybrid. It's a cool idea. I also doubt Nintendo would make the stuff that often goes into amiibo, without amiibo, as they don't really seem to ever fit as standalone DLC.
Like I said, I see it as, it either exists as it does, or it doesn't exist at all. I'd rather it exist.