Think about what threatens you in the job market, and answer the question honestly.
Younger, eager entry level kids who will do your job for 1/2 the price?
Older more experienced employees who know the job inside out and can burn you in an instant?
Being under qualified for the job you're hiring into?
Those are all reasonable threats, but probably not ones you'd care to
share with the interviewer. I'd make up some BS about the economy being weak
and being concerned with the stability of my IRA and 401K.