This is a pretty interesting point. A few things I didn't think about at all.
However, it's still not the same. I know you weren't arguing such but it's dissimilar enough to make your comparisons off.
These aren't just codes, there's people like me who want them for display/collection purposes before whatever DLC they unlocked was even a factor. In addition to whatever features they unlock on whatever game? They might hold more tangible value than any code ever could "Oh they are definitely going to work for x and y game that isn't even announced right now". These are quite literally bonuses in that regard, unless people are buying them on "potential" or something. I hadn't even intended on buying S.T.E.A.M. I may not buy whatever other games than use them when they come out.
Also, as we just saw, every time they come out with a "big" (lol?) release that uses them, they seem to be interested in reprinting them if they are rare. So while i can certainly see the possibility of it being limited/hard to find depending on the time you are looking (like now), in reality most people who want them should be able to get them. And my figure will be a lot nicer 10 years from now than any DLC code for a game I don't play anymore.
There are definitely positives and negatives, but I can't accept that they are all bad from a general consumer standpoint, or even majorly bad. Just different (and different in a way that i quite like, at least right now. Figurines that are both physically appealing and functional are...not common). Maybe kinda like how digital and physical games have different things that someone may like or dislike more depending on who they are. I'm ok with this.
Edit: the more that I think about it the more that I think Nintendo could actually be far more devious than they are being with this stuff. "Peach amiibos aren't selling? Peach DLC in next Mario platformer, clear that stock!" and reward 'loyal Nintendo nerds that bought all that shit already. Granted, that is still not out of the realm of possibility for the future.