But you see, the marginal benefit of individual action is always low, even if there are others that are collectively acting. It pays to defect and let those other people do the job.

That's easily resolved by being a kind of person who experiences empathy involuntarily, and finds it hard to ignore injustice let alone participate in it. This makes defection more expensive than cooperation.

No, that doesn't easily resolve it. Resolving it would require everyone else (or, at least, a large fraction of them) to be like that. This is basically solving social problems by imagining humanity is other than it is.

The only way to solve collective action problems in the real world is via coercion. That is, government.

I can think of several other approaches. Duty, honour, artificial incentives, constructive competition… People do work together without being forced to, you know.