"There isn't a single Czech living today, who doesn't have German ancestors."
That's false, but I don't think anybody cares anyway.
My family colonized Sudetes once the Germans were gone, and people didn't intermary with Germans nearly as much as you seem to think. I know my family tree for about two centuries back and there wasn't a single German there, nor a person with a slavicized German name. As I noted earlier, however, even if there were any, what'd it matter? If you discovered that your great-grandfather was Russian, would it impact your opinion of Russia?
That the current population of Bohemia is the result of mixing of all peoples who have lived there together during the last thousand years is common sense. There's no such thing as a pure Czech, a pure German. Those are linguistic-cultural categories more than anything else. It's as if someone believed that are Irish people who remain pure-blooded Celts from 1000 years ago. In reality, they are the result of intermarriage between the original Gaels, Anglo-Normans, who invaded Ireland, and the English and Scots, who colonised it.
I'll oblige you with genetic data, since you do not seem to think so.
We can trace historical genetic changes in populations by tracing Y haplogroups, genes which are tied to patrilineal ancestry. In the Czech population, approximately 33% carry the typically Germanic haplogroups R1b and I1. Around 57% carry the typically Slavic haplogroups R1a and I2. The rest is divided among rare haplogroups from all over the world.
Every third Czech carries a Germanic haplogroup. Czechs have not practised incest and endogamy to keep the blood of Forefather Čech pure during the last millenium.