Not at all. The private insurers limit the risk pool by excluding stuff, or denying people. The public risk pool is the vast majority of the 80 million Germans. And the whole health care system is basically built around this system. Nothing is becoming lopsided, and nobody is sacrificed, because the public coverage is really good.
EDIT: Regarding public coverage, the only supplemental insurances that are actually really useful are extended pay beyond the legal limit of 6 weeks per diagnosis and additional dental coverage. Other than that, I cannot imagine a worse case than cancer, and I got chief surgeon treatment 2 weeks after the diagnosis while being in under public insurance. So the system works just fine, even in some of the worst imaginable cases.
EDIT: Regarding public coverage, the only supplemental insurances that are actually really useful are extended pay beyond the legal limit of 6 weeks per diagnosis and additional dental coverage. Other than that, I cannot imagine a worse case than cancer, and I got chief surgeon treatment 2 weeks after the diagnosis while being in under public insurance. So the system works just fine, even in some of the worst imaginable cases.