I have been fired once, and the term was "let go" when I was almost 60 years old. I did find a part-time job at a department store, but being 60 with bad knees did not go together. Other jobs that fit my skill set because there is a new trend that you should change jobs about every seven years. I had my job for 23 years and learned new skills every year. To more directly answer your question the fired employee is a liability for the following reasons:
1. Salary has gotten too high.
2. He has more experience and knowledge of the operations of the company than the new President or CEO that has been hired.
3. He is aging and having increasing health problems causing a strain on the company's group insurance plan (the ACA may make that issue mute).
4. He is not a yes man--this is what did me in. My new boss said if he wanted something done he did not want to hear what was done in the past. Being the one with the institutional knowledge, I said, I, though I would have an obligation to tell him if a pass effort had been successful or a failure and why. I also said that would be my opinion, and he could ignore it or use it, but at the end of the day; I would have done whatever he wanted accomplished. I always kept that promise. I was frequently correct. He did not like that.