The name Singleman also suggests that the octogenarian is literally single. He is a confirmed bachelor. He has been living in hotel rooms all his life and likes his rootless, homeless lifestyle. He doesn't have Willy's responsibilities or money problems. If Singleman travels on an expense account, he doesn't even have to worry about his own food and shelter, whereas Willy has to support a wife and two children and has been making mortgage payments on a house for much of his working life.
Dave Singleman is also a single, unique example just because he has lived to be eighty-four years old. According to the actuarial tables, his life expectancy when he was born would have been around 53 years. Willy probably met him some thirty years earlier and Singleman was 84 then! Singleman is a poor model for Willy to follow. The main thing that impressed Willy was that a man could still be traveling and selling into his mid-eighties. That has made Willy believe that he doesn't have to worry about the future because he can go on being a traveling salesman long past the usual retirement age. But Willy has different genes. He is already used up at age 63 and ready for the "ash can."
Willy probably never wanted to retire. A lot of men are like that--especially businessmen. All they know is business. They don't have hobbies. They don't read much except for newspapers. They can't imagine just sitting around the house with nothing to do. Many men create a problem for their wives when they retire, either because they are forced to retire or because they expect retirement to be enjoyable. Their wives are used to having the home to themselves during the weekdays. Their husbands are just underfoot and interfere with their wives' customary routines. Instead of dreaming of retirement, Willy dreams of living like Dave Singleman, having built up such a large circle of business acquaintances that he can just sit in a hotel room with his slippers on and take orders over the phone.