Willy Loman bears a large share of responsibility for what happens to him. While the world outside is changing, he's chosen to remain trapped in a delusional fantasy world of his own making. In Willy's mind he's still a hot-shot salesman. Sure, life may be a little tough now, but that's just temporary; all Willy needs is a change of luck and he'll be back on top again.
Because Willy lives in a parallel universe, he's unable and unwilling to make the necessary changes to his family and professional life. He'd much rather fantasize about success than actually take practical steps to achieve it. When Bernard generously offers him a job, Willy turns it down, not just out of pride but because he's placed all his hopes on Biff's becoming a success in life, a "well-liked man." Yet this is another example of Willy's delusional nature. He effectively hobbled Biff's plans for success in life by not encouraging him to do well in school. This led to his flunking his classes, precipitating a downward spiral of failure and dead-end jobs from which he's never been able to recover. The breakdown between Willy and Biff's relationship is a key factor in Willy's eventual demise, and Willy himself is largely responsible for both.