I think that NFL quarterbacks take as much responsibility as they should. It isn't only the QB that makes or breaks the team. If the offensive line doesn't do their job, the QB gets sacked. If the RB/WR doesn't play as they should, the ball isn't going to get down the field. And if the defense comes in under par, the other team is simply going to outscore you. The QB is a big part of the team, but there are many other factors at play.
I do not think that NFL quarterbacks in this day and age take less responsibility for their losses than they did a decade ago, or than they ever have. Many quarterbacks like Payton Manning call their own plays from the line, and really do hold a great deal of responsibility if the team wins or loses.
A lot of quarterbacks in this day and age do take responsibility but they do it by saying things like, "we could have done [this] better." They use "we"s more because the loss of a game does not rely solely on the quarterback's performance. It is a team sport and each player has to take some responsibility for the loss of the team as a whole.
Whenever somebody's ready to bash an NFL team who recently lost a game, the quarterback is usually the most blamed regardless of the actual play. A lot of offensive responsibility is on the quarterback's shoulders (with calling plays and having to throw to receivers), and the offensive is when the points are scored.