When Michael Bay revealed plans in 2012 to recast the Teenage Mutant Ninja Turtles as “lovable aliens” (rather than, you know, mutants), the internet exploded in a tumult of disgust. There was palpable distress when Batfleck started murdering Gotham’s bad guys with giant Batguns in Batman v Superman: Dawn of Justice, while Ryan Reynolds’ brief cameo as a mute, weirdly powered Deadpool in 2009’s X-Men Origins: Wolverine had fans of the wise-cracking mutant screaming at the screen in horror.

Canon matters in pop culture, and film-makers who play fast and loose with comic-book idols are likely to be accused of failing to treat their material with the respect demanded by fans. At the very least, studios had better have excellent reasons for changing essential aspects of an iconic property’s personality or backstory without due consideration for the regard they are held in by their core audience.

Unfortunately, this perspective is now increasingly being used to argue that the white ethnic makeup of much-loved characters falls within the definition of canon, and should therefore be protected by Hollywood at all costs.

Last year, there was an outbreak of irritability with the decision to cast Michael B Jordan as a black Johnny Storm in the latest Fantastic Four film. Earlier this year, JK Rowling was forced to (proudly) defend her decision to cast a black actor, Noma Dumezweni, as the all-grown-up Hermione in the play Harry Potter and the Cursed Child. And over the weekend the news that the mixed-race actor Zendaya will play Peter Parker’s on-off ladyfriend Mary Jane Watson in the new Spider-Man movie was met by the predictable storm of myopic protest on Twitter.

Since Mary Jane is being played by a Black woman, can MLK be played by a White man in a next movie about him? — Jason Bergkamp (@keksec__org) August 19, 2016

Zendaya being the next Mary Jane is just wrong. Mary Jane is not black. Why can't Hollywood ever stick to the story? Smh — Zoltan (@ricozoltan1991) August 19, 2016

Here’s the thing. The idea that race is an essential part of canon can itself be a racist attitude.

Hang on, I hear you say. The Guardian was all over Marvel when the studio shifted the cultural identity of mystic mentor the Ancient One from Tibetan to Celtic in the forthcoming Doctor Strange movie. And you’d be right: that story permeated the noise of the everyday news cycle because roles written for Asian actors are few and far between in Hollywood, and here was an obvious example of one being whitewashed for the convenience of the studio - in this case, apparently to stop the Chinese government throwing a wobbler.

The situation is rather different when a traditionally white character becomes black. Let’s imagine, for a moment, that Will Smith has been cast as the next Batman. The internet explodes in a puddle of choleric irascibility – but why shouldn’t Bruce Wayne be African-American? The obvious answer, from a deeply racist point of view, might be that the historically lower economic status of black people in the US makes Smith less suitable to play the scion of a super-rich, upper-class American family.

The US’s first black billionaire is generally considered to have been BET founder Robert J Johnson, who is 70. Smith, at 47, is easily young enough to be his son. So why shouldn’t the Hollywood A-lister play the heir to the fortune of a prominent businessman and philanthropist in 2016? And with the realism argument torpedoed, what reason do we have (beyond tradition) to suggest that Batman cannot be played by a black man?

Let’s look at some of the examples above. The Teenage Mutant Ninja Turtles cannot be aliens because removing their Earthly origins story destroys everything that makes the crime-fighting quartet fun. The turtles’ New York patois, Renaissance-inspired monikers and even love of pizza all stem from a deeply ingrained terrestrial cultural identity.

Likewise, fans got upset at Batfleck murdering his enemies because the Dark Knight – at least in recent decades – has always avoided killing, even to the point of putting his own life at risk. Deadpool fans wondered why the garrulous antihero known as “the Merc with a Mouth” had become just another common-or-garden bad guy, his core personality entirely removed when film-makers chose to sew his lips shut.

But making a character African-American doesn’t alter any essential, unshiftable aspects of their personality, unless you happen to believe that black people are so different from their white counterparts that the mere switch in race amounts to a radical transformation. Even asking a black actor to play one of the Ninja Turtles presents no obvious issue, since the loveable heroes in a half-shell were not white in the first place.

The same goes for Zendaya’s reported casting as Mary Jane, a character previously played on the big screen by the American-German actor Kirsten Dunst. If, when Spider-Man: Homecoming hits cinemas, you are unable to enjoy the movie because Parker’s sometime paramour does not have Celtic or Germanic roots (the traditional sources of the genetic mutation that leads to red hair in white people), maybe you need to ask yourself why. Was there really something about that character in the comic books or movies that was so essentially white? Come on – Dunst isn’t even a natural redhead.

Moreover, Spidey, as Captain America: Civil War reminded us, hails from Queens, which according to the 2014 US census is hardly a bastion of white America.

Back in 1950, the closest census to Spider-Man’s 1962 debut in the comics, the non-Hispanic white population of the borough was 96.5%. In 2014, the most recent census, it was just 26.2% – a lower proportion than those who considered themselves Hispanic or Latino, and roughly on a par with the borough’s Asian population, which stood at 25.8%. Black people, for the record, made up 20.8% of the population. So when Peter Parker looks over the backyard fence in 2016, it makes perfect sense that the girl next door is a person of colour.

If, on the other hand, the new Spider-Man is revealed to have gained his superpowers after picking up a power ring belonging to a race of sentient arachnid aliens – rather than being bitten by a radioactive spider – or is from LA rather than New York, feel free to see how much rage you can pack into 140 characters. But getting upset because a previously white character is now being played by a black person? That’s really not something that should be happening in 2016.