That theory would make it a rather more boring movie though. The unexpected reversal of fortune at the end there is a great ending.
I’ve always read it like that, though. Minus the alt-right movement, but still. Bickle is a crazy person and we expect things to go badly for him - we can see the story arc ending badly during the whole movie, like a true Greek tragedy - we just wait to see how he is destroyed. Only he isn’t, ironically.
Scorceses did something very similar in King of Comedy, by the way, and it works brilliantly there, as well. We expect Rupert to be a terrible comedian and to fail miserably; the whole movie generates that expectation - when he turns out to actually be great, that’s an incredible surprise and turns the movie in a wholly unexpected direction.