Some classified documents may contain information that, when made public, would lead to greater harm than good. For instance: military secrets, intelligence operation details, embarrassing personal correspondence, security measures on critical infrastructure, enemy propaganda, etc.
This premise is nowhere near specific enough. For instance, one ought to know *when* they will declassify them. Or whether the time limit is different for military documents than civilian (hint: it likely should be). Or if they'd be redacted when they were declassified.