One of the things that few Germans grasp is that there is a vast and wide landscape of nationalities and cultures in Germany today (more than eighty nationalities).
They range from the Turks with roughly 1.5-million all the way to down to Liechtensteiners with roughly 240 folks.
Some come to study or attend college (like a lot of Chinese). Some come for safety (like the Syrians and Iraqis). Some come for jobs (like the Turks).
Just because they have a visa to work, study or stay long-term....does NOT mean that they are actively seeking citizenship.
Roughly eighty-percent of the German society (82-million) are native full-blown Germans....which means that twenty-percent (around 16-million) either were born else-where and became a German, or have a visa to visit or stay long-term.
The 90,000 folks from Vietnam? The mass majority were part of an exchange program with DDR (the old East Germany) and were brought in for work-assignments. When the wall went down....no one said much, and it's believed that most stayed and integrated into Germany.
The South Koreans in Germany? They number around 32,000. Their introduction into German society started in the early 1960s as part of a work-guest program (mostly for miners and nurses). While the program had a end-date and end-date to it....as the end-date approached....roughly half of the South Koreans applied to stay permanently in Germany. One of the interesting aspects of the Koreans who stayed....their next generation (the 2nd generation) had almost 70-percent of the kids reach Abi-level (meaning 12 years of high school and a year of technical college). If you look at normal German statistics...they typically reached the 35-to-40 percent range....so the Koreans-turned-into-Germans put a lot of emphasis on their kids to achieve and move ahead in the German system. The Koreans have a very large presence in the Rhine-Main region (Frankfurt), with over 6,000 in the region.
If you go and ask most Germans about having foreigners around....they will typically cite the Turks, the Greeks, the Russians, and the Italians. They will typically point out the Greek and Italian restaurants in the local area as an example of successful immigrants.
As much as some folks do 'dump' on the immigration policy....the trend up until the 2013 period was mostly positive and with minimum complaints. The only way that West Germany and later...Germany itself....reached economic success and built up dozens of known companies....was the effort to get guest-workers into the country and fast-pace them along the way to production and integration. If you went and asked most who came into Germany in the 1960s or 1970s....they will say that a basic course in German occurred, and some German co-workers pointed out a few things along the way. Otherwise, integration was almost non-existent in terms of a program existing.
What happened along the way? I think there was always a magical number in the air in terms of people coming in and the pace that they took. You won't find anyone who will say what the magical number is, or how they arrived at the magical number. PhD folks and intellectuals will mostly just grin because you can't build a science or logic around this magic number. The CSU of Bavaria will tell you that the magic number is 200,000 per year. They can't cite any science behind that number though. The pro-asylum folks will say that the number is unlimited, but they can't cite any science behind that logic either.
Adding to this conflict is the fact that integration is continually argued about. If you offered a class and a hundred question quiz....then offered the same test to some fifty-year-old Germans (folks who've been here their entire life)....probably a quarter of them would fail the German integration test.
So you are left with a door that must be forever left unlocked and somewhat open....at least in the minds of the German leadership.