You bring up a fair point, except that Pym is a weird character in the comics. For years writers had a hard time writing for him, and he never had a very charismatic or interesting personality to many. And one of the few things he is known for, personality-wise, is beating on his wife at one point--not an enduring trait. So, unlike Johnny, I can imagine more fans of comics being a bit happy inside to see Pym go, just because he's kind of a strange duck no one cares much about.It's funny people are more upset about the Johnny Storm casting than they are with Hank Pym (if they're even upset) in Ant Man, if you ask me MBJ has a better chance at conveying all the essential characteristics of Johnny Storm from the ultimate comics than Michael Doulas can for Hank Pym. Making Hank Pym like 90 years old is a much bigger change than making Johnny Storm black. I have to wonder why there isn't an uproar over what Marvel's doing with Ant Man, which is looking like a larger deviation from the source material than F4 is right now. I don't think it has anything to do with racism, I just think most people on here are blind Marvel fanboys that'll eat up anything they serve.
I actually think it makes a lot of sense to focus on Lang simply because of the other decisions made in Marvel Studios thus far--in the comic, one of Pym's defining traits is that he's not a fighting hero at heart, but a brilliant scientist. Avengers already has two brilliant scientists, so sticking an accurate Hank Pym on the team would be bad for the story.
So I can see both a very legitimate reason from a storytelling POV, and a legitimate reason from the fan's POV, for not wanting Pym. Johnny Storm, on the other hand, is much beloved and distinctive in the Marvel Universe. And the FF movies aren't tied down by any previous films. Taking liberties with him does seem more arbitrary, and can understandably bug some fans of the classic comics, even if I don't agree with those people.