> The problem is you can´t easily opt back into the public system once you´re out (especially once you´re old), so there are tens of thousands (if not hundreds), that get stuck in the private system without being able to pay the fees because they´ve become poor (possibly because of health reasons), and then they get denied coverage and thus access to medical services.

Although I recognize the personal hardship this can cause, I fail to accept this as a general injustice. In the German healthcare system nobody is forced to choose a private insurance instead of the public one. Choosing a private insurance when you're allowed to (which often means because you're in a higher income bracket) is usually only cheaper if you're young and healthy. It's basically a gamble on your own health for wealthy people.

And if the wealthy young and healthy population don't contribute to the public health insurance system which also pays for the not-so-wealthy and not-so-healthy (older) population, the public insurance system isn't sustainable.

The real problem with this system is its division in public and private insurance. Mandatory public insurance would mean lower insurance fees for public insurance and the problem of being stuck in private insurance would no longer exist.