You are putting way too much stock into what the Amiibos add to the game.
First of all you have to complete the single player campaign missions to access them in Amiibo mode, in Amiibo mode you do not get level cohesion, a feeling of progression or story, or anything like that simply because it's only 3 levels (out of 6) from each story area and the boss fight. For the most part (excluding the very few [less than 5] limited ink challenges) you just rush your way through the levels as quickly as possible. All of the squid amiibo challenges can be done in about 45 minutes.
The gear you get is basically useless as soon as you are over level 10, and the weapons are simple reskins.
The only reward that will have continued value are the mini games, and let's be real for a second, you didn't buy Splatoon to spend your time in the plaza playing some 2 button, extremely repetitive NES games, and the longest I've had to wait in the lobby is 30 seconds. I haven't even been able to get a quarter way through 1 song on the music mini-game.
Amiibo challenges don't unlock variety, they unlock anger and frustration. Squid boy challenges make you use the Roller, Squid girl challenges make you use the Charger; The Charger is the least popular weapon in the game, the Roller makes some of the challenges nearly impossible and the only reason I stuck with them (instead of throwing my gamepad out the window) is because I'm a completionist.
Honestly, they really don't add much to the game at all besides some extra mini games you might get to play 10 seconds of in between online matches. The gear is only 2 star and is replaced pretty much as soon as you hit level 10 (a few hours play time), and most of the challenges were anything but fun. If you don't have the Amiibos you really aren't missing out on anything unless you really want that cosmetic reskin of the never used regular Splattershot.
TL;DR Nintendo probably shouldn't have put what seems like a lot of content behind Amiibo paywalls, but at the end of the day Nintendo wanted to make it seem like it was a lot of content to try and sell the Amiibos. In actuality you are getting around 3 hours of frustrating and fury-inducing single player content you have already completed in a more balanced way, some useless gear you'll never use after a few hours of online play, 3 weapon skins, and 3 2 button NES games you won't ever get to play.