How does written law enforce itself if the police and the judges are compromised?

Everything is ultimately enforced by people. If people stop believing in something, the government will eventually follow suit. And not just the handful of top leaders elected or appointed for a few years, but most people from the top to the bottom in every branch of the government. Especially the ones with the power to make a difference.

The written law may say something, but people in power are very good at twisting its purpose and ignoring it. Especially when that's something everyone expects from you.

My point was that an oppressive government cannot appear out of nowhere in a free country. The citizens must abandon constitutional values first. If an oppressive government remains in power and maintains its popularity long enough, it will infiltrate all levels of the government and compromise checks and balances.

Then, with popular and institutional support, the government can do basically whatever it wants. Regardless of what powers it had before or what the constitution says.

You should not be afraid of giving the government new powers simply because it might go bad later. (There are other valid reasons, but that's not one of them.) If the government does go bad, it can take those powers on its own just fine. You should be afraid of your fellow citizens going bad and starting to think that their personal goals and values are more important than constitutional values. Because that's a prerequisite for the government going bad.