This is a political question and the answer depends on broader ideological concepts about the role of the employer in a society. These are exactly the ideas Steinbeck means to call into question. What is the role of the employer? Do the ranch owners have a moral obligation to Candy?...
This is a political question and the answer depends on broader ideological concepts about the role of the employer in a society. These are exactly the ideas Steinbeck means to call into question. What is the role of the employer? Do the ranch owners have a moral obligation to Candy? Clearly, they have profited from his labor for many years. They may feel in the clear with him because they compensated him with $250 (about $2,500 in today's money) for his lost hand, but obviously that isn't enough money for him to achieve financial independence or survive many years of old age.
Candy is deeply worried. Social Security was enacted a few years before the time of the novel, but domestics and farm workers were exempted from it, so Candy can't count on a pension. The question becomes ethical: especially in the absence of a government safety net, is a long-term employer under a moral obligation to take care of an aged employee? Clearly, as others have pointed out, Candy is doing useful work for the ranch at this time, so they are keeping him on. But is it fair to get rid of him once he can't work?
Our society trends in the direction of individualism against corporate responsibility toward the worker, and the same was true in the 1930s, which is why Steinbeck is writing the novel: he wants to critique the capitalist system. Certainly, part of the power of a novel, especially a great novel, is it's ability to cause us to empathize with its characters. While it is easy to make ideological statements, a novel about social justice, be it Uncle Tom's Cabin or Of Mice and Men, puts a human face on abstract principals. Up close and personal, I would argue that Steinbeck causes us to feel Candy's pain and to feel that a person who has faithfully worked for so long and so hard, and lost a hand in the process is entitled to be kept on and not fired--and one might venture to say, even offered a retirement pension. Firing him would be exceptionally cruel, given his age and lack of a hand: one could imagine he would not be able to compete successfully for another position.