I think the problem here is Nintendo themselves.
We need that Amiibo Game for the NX at Launch!

I don't care for those companies. I care for Nintendo. I don't want to buy "figurines" to be able to unlock the water tunic for Link or the gravity suit for Samus. Unfortunately, Wolf Link is a treat for my delusional dream where I have access to all the content on the disk I already paid without the need to buy a "collectible".

Bear in mind that Nintendo are not selling any version of Amiibo Festival without Amiibos, so DLC wouldn't be the right word to describe the Isabelle and Digby amiibos in that game.

I think he's saying that the main consumer segment is taking them home and putting them on a shelf.  Obviously it's good that they're selling but they seem to want people to use them and show off the "DLC" aspect to others, who would then go out and buy their own Amiibo.  Basically, they want Amiibo to sell to people who are interested in the functionality, not just super-fans who buy for their collection.  One is a path to growth, the other not as much.

I think they wanted Amiibo to go viral and pull in new customers.  They wanted that Skylanders effect.  Kids, especially, will buy what other kids are playing with and your product adoption rate will grow exponentially.  My guess is that growth is more flat than hoped for and they're looking at 20 year old hardcore Nintendo fans buying them all up rather than 12 year olds who become new fans because of Amiibo exposure.

If I want to play with another, let's say, 2 characters more?

And what we can guess that they are going to do to reach this goal (because you explained very well)? Let's lock more content behind it, let's develop more games that needs them, let's focus more resources in amiibos, etc.

This type of mentality is grossly misleading.  What you paid for isn't every scrap of data on the disc.  What you paid for is the initial vision of the game that they put out.  If they have additional content on the disc that is REQUIRED for you to play and ultimately complete the game then their actions would indeed be abhorrent.  However, I have yet to see paywall gamehalting DLC on the disc on any game from any company, ever.

However, if you are going to tell me that your gaming experience is ruined because you can't get a specific skin that they locked behind the amiibo then you might have the wrong priorities in what you want in a game.

I already paid for that custom. Why do I need to pay for it again?

No, you actually didn't.  That's the part you are missing on this.  You feel entitled to the full content of a disc when the reality is that the developer of the software decides what you get access to and why.

Really, DLC is a smart bussines.