45 years after the Pentagon Papers, a new challenge to government secrecy

Significant to who? A very good argument can be made that there is absolutely no good reason for our government to keep ANYTHING secret. Any behavior the government thinks that they need to keep secret, they should not be engaged in. The harm done to our country and our "democracy" by the secret (and often illegal and unconstitutional) actions of unaccountable government officials is far, far worse then any claimed benefit.

The loss of these secrets would seriously weaken the U.S. in diplomacy since adversaries would know the exact limits and shortfalls in U.S. military power.

Most secrets, as you rightly assume, aren't in this class. Most of them are analysis literally gleaned from the open internet and combined in a report to a policy maker.

I don't think the classification system is a good repository for secret corruption. Tenured federal employees have little reason to hide the impropriety of an elected official; on the contrary, they can get a cash bonus for reporting the malfeasance! In the rare cases when corruption was masked by military secrets, e.g. the Iran Contra affair, the profits were small (in the tens of thousands) compared to the scale of the wrongdoing (the undermining of the rule of law, Congress, and the people).

The best way to detect corruption would be to have better aggregation of officials' assets and analysts dedicated to the task. Elected officials and security clearance holders currently fill out an annual financial disclosure, but corruption can easily be obscured from the process.

- Human intelligence assets would be executed, as you mention. Witness protection program participants, too.

- Signals intelligence targets would stop using (or feed false information to) compromised systems.

- Consequently, the government would lose what little understanding it has of what's going on in the world, threats, likely outcomes of foreign policy, etc.

- It would also be incapable of any sort of undercover/sting operation, because every target of an investigation knows they are the target of an investigation.

- Diplomats can no longer talk about 3rd parties behind their back. No more conspiracies between allies to keep another nation (i.e. North Korea) in check. No more back channels.

- Diplomats must negotiate with foreign leaders in full view of their populations and political opponents. No more reasoning with leaders, brokering secret face-saving solutions. (Giving foreign nations an opportunity to back down without appearing weak is a staple of diplomacy).

- The budget required to replicate our military technology (including nuclear arsenal) shrinks dramatically, amplifying threats currently below the threshold of "serious."

- The nature of our defenses and locations of troops, installations, ships, etc. becomes public, lowering the bar to a successful attack.

- Some of the government's secrets are your secrets too. The full name/DOB/SSN mapping used to authenticate yourself to financial services. Home and family addresses for federal employees like prison guards, judges, and prosecutors.

- We've blown confidentiality, but some systems are not read-only. Properties like integrity, authorization, etc. often depend on the confidentiality of passwords and private keys. When the admin passwords enter public record, we have to assume all government databases and infrastructure (taxation, payroll, Exchange servers, PBXes for federal offices, etc) are promptly trashed by bored teenagers (or even better, overwritten with subtly wrong data/configuration for the lulz). How long before the email password of the Secretary of Defense is used to announce a foreign policy change? How long do you think the signing keys for a valid nuclear launch order will sit on Wikileaks before someone uses them?

- Do we count the federal reserve? Authentication factors needed to make arbitrary bank transfers are out in the open too.

Embarrassment is a weird thing in politics.