FEWER THAN

FEWER THAN is correct idiom just as LESS THAN, GREATER THAN etc.

If the draft is not re-instated, less people will join the army in the coming 10 years than did in any other 10-year period in our nation's history.
A. Same
B. less people will be joining the army in the coming 10 years as
C. Fewer people will join the army in the coming 10 years as
D. Fewer people will be joining the army in the coming 10 years as
E. Fewer people will join the army in the coming 10 years than

"Fewer...as" is not the correct idiom when there is comparison, as is used for showing similarity so only option E is correct.