Taxi Driver is one of the best movies of all time. Every movie from the neo-noir genre is worse from it by default. Phillips in no moment ever tried to deny any kind of inspiration or homage he did. And a comic book story taking from classic cinema is not new. Long Halloween takes a lot from The Godfather. So much in fact Alberto Falcone is basically Fredo. Year One homages many points and scenes from Le Samourai (most of Miller's work, in fact). Arkham Asylum, A Serious Home in Serious Earth, which is basically a mix of The Shining with The Wizard of Oz and Psycho. The Batman homages a lot of Se7en, Zodiac, and Chinatown. The Dark Knight Rises from "A Tale of Two Cities". Hell, the character of the Joker himself is taken from The Man Who Laughs from 1928 and Batman from Zorro, Sherlock Holmes novels, and The Shadow. Are these stories bad? No, they're still good stories that everyone loves, including me, because they execute their own tale while telling said story.
Many stories from this genre are built in homages and inspiration. Even then, the character of Fleck is nothing like Bickle. And if you think this then you need to rewatch both stories, considering both suffer from a different illness and a different story. Fleck being a product of repression, childhood trauma, lack of fatherly figure, and an imposed delusion by her mother, and Bickle a product of war, trauma, and rage. I'm not saying there are not similarities, it's an homage, not only to Taxi Driver but also to Network, Modern Times, Excalibur (The Arthur name coming from there), etc. I'm just saying it's not new in this medium to see that, and both stories have different conclusion and execution. Hell, even Taxi Driver itself is a mix of Le Samourai with The Searchers (something Schrader confirmed, considering the story it's a deconstruction of the "american hero" portrayed in westerns).