James absolutely started the rivalry by interrupting Snape's private conversation to be rude about Slytherin - but Snape was already showing signs of believing in blood supremacy (telling Petunia he wasn't talking to her because she was a muggle, dropping a tree branch on her, having an internal struggle when Lily asked if it mattered that she was muggleborn, and telling her not to be upset that Petunia hated her because she was only a muggle). Sirius describes him as and oddball who came to school knowing more curses than most seventh years. He was always very drawn to the Dark Arts. They never laugh at him for being poor (and it's unfair to say they do). They do laugh at his greasy hair (but he still has that as an adult, so is just unfortunate as opposed to a sign of neglect, in fact we don't know he is being neglected - he bears signs of poverty but that doesn't mean his mother is not doing her best). Yes they are obnoxious (and Sirius admits it) but overall it is their opposing views which lead them to target him.
Snape comes from a poor family, and we know his father was abusive towards his mother. But within the wizarding world he is not marginalised, even as a child. He is a half blood, his mother had a good wizarding name. These are the things that matter among those who discriminate in his world. He doesn't come from the easiest background, but he is nothing like Remus. And while he is a victim of the domestic abuse going on his home (even if only by witnessing it) his decision to seek power by pursuing the dark arts and joining Lord Voldemort is all his. The older they get, the more obvious his path into darkness becomes. James and Sirius don't know about his home life and probably don't care much about the state of his finances. They do know he calls muggleborns "mudblood", sneaks around trying to get them expelled and tried to expose Remus as a werewolf. That's what they don't like about him.
While they are not actively fighting the war at school, it is already going on and Snape has chosen his side. Only hours after James dangles him upside down Lily says to him "you can't wait to get out there and join You Know Who" she refers to his friends as "Death Eaters". Mulciber has already used Dark Magic on Mary and Snape has defended it as "just a laugh". Absolutely their world was at war and James and Snape were on different sides of it and knew it.
SWM is an important point of growth for Harry. Up until now he has idealised his father, seen him as a shining hero of goodness. Now he sees his father in a moment of cruelty that appals him and it tilts his whole world on its axis. Learning to come to terms with the fact that James wasn't perfect, that all idols have their flaws, is a huge part of Harry growing up.
Yes, when SWM happens - if it is shown in the detail it is given in the book - it will make James look bad. It is supposed to. But over the next couple of books/ seasons - as we learn that Levicorpus is Snape's spell and people used it on each other all the time that year, that Snape was best friends/ in love with a muggleborn but still wanted to become a Death Eater, and that the incident with James is not his worst memory because he got hung upside down but because it is the cause of the destruction of his friendship with Lily - the whole thing will get put into a wider context. The scene will get visited again, within that context ... and its just a bit of hexing (I recently wrote that scene for my fanfic, the spells they use are no worse than anything Harry would do to Draco while Snape attempts to use Sectumsempra on James).
Will this one tiny scene all the way in the fifth season look worse because Snape is black? Yeah OK, probably. But is that a reason for all this handwringing? No. It's one scene. Does it mean they should actively make this one tiny scene less "problematic" by making one of the marauders a POC? No. If the best person for the job is a POC, that's great, but otherwise the scene can stand as it is, as can Snape's and their well documented rivalry and dislike of each other.
Viewers can simultaneously accept that within our world what we are seeing (in this one, little scene) looks like it has racist undertones and that within the wizarding world it doesn't. Viewers of Buffy watched a woman get punched in the face all the time, often by her love interest. We accepted it because it was fantasy. Saying "this makes James look racist" is reactionary and ignoring the fact that this isn't set in our world and that the discrimination in the wizarding world is along totally different lines. These viewers will always exist, but then they will always find something to react negatively against while ignoring context.
It isn't the end of the world if viewers feel uncomfortable watching the scene (heck, they are supposed to). But intelligent viewers will be able to understand that they are feeling personal discomfort that is not relevant to the story they are watching (as Buffy viewers ignored all the times Spike hit her in the face - though admittedly the attempted rape is still controversial nearly 25 years later). When they finally get there, we can all go to town on discussing whether they did it well or not, whether they managed it (or has it gone down like the attempted rape in Buffy) but, unless the have James saying the N word, in world it still won't make him a racist and it will only be disingenuous people who try to pretend it does.
Yes, gender and race swapping isn't always a straightforward process, and can change a story significantly. But in the first few HP books there is nothing that is significantly changed by Snape being black. It is only an issue in that one scene in ootp. So what? Having James Potter look marginally worse than he already does in that scene, when you see it for the first time devoid of context, is a small price to pay for having the best person in the role of Snape, and a black man playing a prominent role in a culturally significant show - and the knock on effect that will have for actors of colour being given more nuanced roles, getting to be anti-heroes and villains (whose villainy is not linked to their race) and three dimensional characters who just happen to be POC because it has been shown it can be done over on HP, and the world didn't end and the viewers dealt with it.