Would you then suggest that a country of mixed-nationality immigrants, like Canada, can never have a national identity? Do you believe that the patriotic affinity between Canadians is somehow lesser than that between Germans, because we don't share a well-defined genetic heritage?
You're right, Turks have integrated poorly in Germany. Temporary foreign workers were brought over en masse and ghettoized to fuel the economic rejuvenation of the nation; unsurprisingly, many chose to stay. Too many to integrate properly. The result is a huge chasm between German-born Turks and Germans, with resentment on both sides. But this was the result of a poorly-executed policy, albeit one that may have been necessary at the time; either way, it's not the normal way that immigration happens in Germany or other countries.