Well like I said, I'm only considering the perspective of the studio execs and marketing people here, who probably pay very close attention to every little detail (and every bad word said) about their upcoming summer blockbusters.
They undoubtedly want this movie to be a huge, billion dollar hit along the lines of Avengers, and anything that could possibly put that at risk or jeopardize it is probably going to make them a bit nervous-- at least for the time being.
Except that they do pay attention to
every reaction, not just the cross-section that we sci-fi/comics nerds get in our bubble. That's a really small fraction of the total audience, so it doesn't loom remotely as large in the studios' considerations as it does in your or my experience as a fan trawling the Internet. Outside of the geek bubble, Ben Affleck is a highly regarded actor-writer-director, and I imagine a lot of people are bewildered at the intensity of the fan reactions, because
Daredevil does not define Affleck for them.
And just in general, people who pay attention to all the hype must be used to the ridiculous overreactions that
always come from a certain segment of the fan community, so they'd know better than to worry about it. You're talking as though this is something new or unprecedented. It isn't. It's the same thing we've seen time after time after time, and there's no reason it should be any more of a concern now than it was then.
If studios were paranoid enough to overreact to fan rage, then they would've replaced Heath Ledger as the Joker and we would've been cheated out of one of the great performances in film history (although maybe Ledger wouldn't have died then, so, hmm). Fan rage is routinely wrong. Sometimes it's right, but then, if you guess "tails" every single time a coin is flipped, you'll end up right about half the time. Statistically speaking, there is no reason to trust fan rage as a reliable predictor of anything. It's just a reflex action.
I think maybe the only bit of comics-movie casting that every fan immediately loved was Patrick Stewart as Professor X. Heck, everyone who was a fan of both
Star Trek and the
X-Men had believed all along that he would be perfect for the role. A friend of mine in college nominated him for Professor X back in the late '80s. Diane Duane wrote an X-Men novel where Xavier talked like Captain Picard years before Stewart was actually cast in the role.
Even so, there were probably people who went "WTF? Since when was Professor X English?!!!!"