> Does a constitution still matter, if its values are no longer the values of the people?

Yes, otherwise the incumbents could pull stunts like opening the borders to flood the nation with foreigners, radically redefining who "the people" are in order to dictate what "our values" are.

The entire point of written law is to outlive the whims of human nature.

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.