http://www.constitution.org/mac/prince00.htm
The NSA is unable to do a thorough damage assessment -- they don't know how which documents Snowden took.
Greenwald is drip feeding the world press "stories" which can go on for an indeterminate amount of time (see "no damage assessment").
Only viable option is to keep Clapper in place until Greenwald et al. have exhausted their supply of new scandals.
If, for example, Snowden had gone all Wikileaks and dumped the whole lot of files at once, Clapper would have been gone months ago.