In fairness, like several others here have already said, that first movie really seemed like it was gearing him up to be much more important than he ended up being. He was my favorite part of The Force Awakens and I do think he should've been treated more like a main character for the movies that followed, but this wasn't racism, it's pretty obvious the directors had competing visions. It's Disney's fault for episodes 8 and 9 not really following through on what was set up in episode 7 but not every poor decision is rooted in racism, and like everything else when you call everything racism it seems like you're crying wolf and it's harder to take your word for it when something is done that is rooted in racism.
His character's girlfriend has it far worse. They spend all of episode 8 ramping her up as his love interest and then in episode 9 he barely cares about her and is visibly thirsting over Rey the whole movie. Her character is basically irrelevant to anything going on in the trilogy, at least Finn sometimes did things that mattered.