Seriously, why people get too upset about simple, trivial things like green eyes or blue arms?
These characters get a single, individual, iconic look. Part of the reason they resonate with people is because of that design. Mario looks like Mario, forever.
Even if you portray him in a different art style, he still looks like Mario.
There are immutable elements that make him Mario: the shape of his hat, the curly black mustache, the overalls, the white gloves, etc. That's how you know it's Mario. That is his character design, with an emphasis on character. It's part of who Mario is as a person. It is his identity. It is what makes him iconic.
Just like how you know a friend or a family member by their favorite foods or their taste in movies, we connect to a character by the way they look. We'll never know them personally, because they aren't people, but we know the immutable facts of how they look and that forms our attachment to them.
When you change that, you are changing their identity. Imagine if you bond with an uncle over the fact he loves cars, and then suddenly you go to his house one day and he doesn't know what cars are and he doesn't like them. That thing you shared is gone now. That's not going to just roll off of a person.
Similarly, if you change a character's iconic look, you are potentially breaking the bond people share with that character. That can be a dangerous prospect.
I've spoken about this occasionally before, but I think success can be and maybe often is an accident. This is where the idea of the "one hit wonder" comes from. Somebody gets it right one time and they can never replicate that success. Because their success was an accident. Which is fine! It happens to a lot of people! It's sort of like winning the lottery! You still got famous, you still got a lot of money, and there's no shame in that. But it was still an accident.
Changing the iconic look of a character has to be a delicate, carefully considered thing. Being iconic means the character itself is "an icon" -- a symbol of power. Something people draw a kind of strength or inspiration from. It's not something to be discarded lightly. Being an icon is one of the most meaningful definitions a thing can attain. Changing that is not "trivial."
And with regards to Sonic the Hedgehog, Sega has been clumsy about a lot of things, to put it lightly. So when they change things about how he looks, they are threatening his identity.
That's just the start of it. It also creates a hard dividing line where people can attach these changes to other problems inherent in the character. It's the Robotnik vs. Eggman debate. They changed his name to Eggman, I stopped liking the games as much after they did that, ergo, they never should have changed his name to Eggman. The change is what made it bad. Even though that's not necessarily true.
And for me, personally, blue arms upsets me because for the longest time, the number one easiest way to spot a bootleg Sonic was by the blue arms. It screamed "I do not care about being right." It's like those old episodes of Teenage Mutant Ninja Turtles where they had animation errors with duplicate turtles on screen at the same time, or the wrong voice was coming out of an incorrectly colored turtle.
That kind of stuff is an error. The colors are wrong. These characters are designed to look a certain way, that's what makes them iconic, and straying from that is considered incorrect.
By allowing Sonic to be rendered with blue arms, Sega is legitimizing decades of ugly, nasty, cheap, maybe even harmful stuff. It says, like a lot of things Sega has done with Sonic over the years, that maybe they don't really care. Or at the very least, don't really understand. It's more than "It's different, and I don't like things that are different." It's genuinely bad for the brand.
Which leads to the worst part in all of this, and that these changes create discourse. More than a dividing line for fans to say, "changing from Robotnik to Eggman is when everything went wrong", it creates division in the fandom, because you inevitably get people who say "I think the blue arms are better, actually." And now suddenly we have discussion, and people have opinions on things that never mattered before today. The people who say they like Sonic's blue arms never even considered an alternative before and would have remained perfectly content if things were never changed.
But now they have an option. Pandora's box has been opened. You've created a splinter. And that means people will, inevitably, argue. Because, no matter what it is, somebody on the internet is always wrong. Taste is subjective. Even if it's objectively harmful to the brand, or bends rules of good character design practices, people will still say "but I like it and you are wrong."
And when people are told they are wrong, they often take it personally, especially for things that are so subjective. Nintendo just decides one day that Mario is barefoot now, and that immediately escalates in to "What are you, a weirdo who likes feet?" "What do you mean weirdo?" etc.
Retaining iconic designs matter a lot more than I think some people realize.