Biff's assessment of his father's dreams is ultimately validated by the play at large. He argues his father's dreams were wrong because they were shallow, focusing only on a meaningless, empty idea of what a successful life should look like. For Willy Loman, success meant being liked by other people and being financially well-off, making Willy totally at the mercy of other people's validations of his behavior and life.

Personal happiness does not factor into this definition of success, making it hollow. Willy had no passion for salesmanship: he liked the image it gave him more than he liked actually doing the work. Biff notes that Willy seemed to enjoy doing work on the house more than selling products, suggesting Willy could have found happiness had he pursued a line of work closer to his own aptitudes and personality.

In the end, Biff rejects his father's notions of success and sets out to forge his own unconventional path, while his brother Happy is seduced by the same warped idea of the American Dream which consumed and then destroyed Willy. In this way, Biff is presented as the antithesis of his father and allowed a chance at happiness which eluded Willy and will more than likely elude Happy as well.

Biff is correct that Willy had the wrong dreams. This insight shows how Biff has grown and come to see reality clearly. It offers the tragic play a glimmer of hope that Biff can make good use of the insurance money from his father and forge a different path.

A fundamental problem with Willy's dream of becoming a successful salesman was that the dream didn't have anything to with liking or having an affinity for sales. To Willy, selling was simply an easy way to get rich quick. Willy deluded himself that he would quickly become a person sitting in his velvet slippers in a hotel room doing nothing more than answering the phone as the orders rolled in.

Willy also wanted to believe that success could come simply by having a good personality. He didn't want to put the work into to developing any kind of expertise. He didn't want to be a plodder, slowly building to success. Tragically, he foisted his misplaced values onto Biff as well, discouraging him from getting education by pushing the idea that his personality could get him anywhere.

Willy would have been happier and more successful had he pursued a career in gardening or some kind of outdoor work that used his talents. He might never have gotten rich, but he would have found self-respect and satisfaction. As it happens, he ends up neither with wealth nor self-respect.

On the whole, you'd have to say that Biff's assessment is correct. Notice, however, that Biff doesn't criticize his father for having dreams; it's that he had the wrong dreams. It's not that Willy was too ambitious or wanted something better; it's that he had unrealistic dreams that led him to make bad choices in life. Most people have dreams, and that's perfectly healthy and natural. But Willy's dreams are delusions because they're not related to who he really is as a person; they're not an accurate expression of his soul.

A good example of this is the brief affair he had with his secretary. Willy prides himself on being a family man and yet he was so corrupted by his unattainable dreams of success that he cheated on Linda with a woman who ultimately meant nothing to him. His dreams of being a hotshot salesman, a "well-liked man" led him to look upon having an affair with an attractive young woman as one of the trappings of success. And this, in turn, affects Biff. Once he uncovers the affair, the relationship with his father is ruined forever. Willy's dreams have impacted negatively on Biff, so it's not surprising that he regards them as being so terribly wrong.

Biff was correct in his assumption that Willy's dreams were "all wrong". Shortly before Willy's death, Biff began to experience an epiphany in which he realized the extent of Willy's fantasies and the effect they had on him. He noted that Willy lived a life of self-deception and false ideals, always chasing after a victory that Willy himself could not fully describe. Furthermore, Biff knew that his father's talent would have been better invested in chasing after the things that he really loved, which were nature, the outdoors, and building things.

In addition to this, Willy did not become a salesman because he wanted to become one. Instead, he did it because he once heard that a man named Dave Singleman had become successful as a salesman. Therefore, Willy was trying to repeat the success of another person, and did not try to go after his own.

This is also why Biff's assumption that Willy's dreams were all wrong is correct. Not only was Willy not going after what he loved, but he was also going after the dreams and successes of someone else.