It is almost always possible to disclose the information needed for some proper public purpose while safeguarding what must be kept secret. That is not to say it is always easy to do. Sometimes, in lieu of disclosing top-secret information, the government must acknowledge facts it would prefer not to acknowledge. In lieu of, say, identifying a specific confidential informant, the government might admit that it engaged in questionable behavior (such as investigative tactics that were too intrusive under the circumstances that obtained).
That’s an important distinction that has been lost in the public debate. Declassification is not the same thing as disclosure. The fact that an intelligence agency may be directed to declassify information so that investigators are permitted to see it does not necessarily mean the information will be made publicly available. Ultimate investigative conclusions will be made available, but some information (maybe a lot) will be withheld to protect vital intelligence (though made available to Congress — probably through senior bipartisan leadership and the relevant committees with security clearances). Think, for example, of the January 2017 Intelligence Community Assessment of Russia’s interference in the 2016 campaign. Most underlying intelligence remains concealed, but enough was declassified that a public report could be issued explaining the essence of what happened.
To argue that this cannot be done is to say that unelected government officials can do whatever they want, and the people’s representatives cannot question them. That’s dangerous and ridiculous.