Bang on... it's initially about opportunity. But when that runs out, people move on.
And with the offer of DE citizenship where you're not giving up your birth citizenship, most people will take it, and move somewhere else in EU with a shiney new DE passport.
I've been here for a decade, and sadly I feel the issue is upward mobility for skilled workers. Unless you're working for an intl company, with ex-pats in positions of leadership, your chances of "getting ahead" are going to be limited, especially when you're competing against natives.
The reason is sadly, the culture is very reserved and cautious, so as an "outsider" it's going to take A LONG time before you can be trusted in a senior/leadership position (no matter how good your German language skills are).
The good part, from my experience the people here are great, friendly, and yeh it takes time to get to know them but it pays off in the long run. But professionally... it's complicated.
So while people come here, work and stay for a few years, they're going to leave when they realise that despite their best efforts, they need to do 10x more than someone who is simply "a native" to the country (or... you'll stay in a position and just rot until you move on).
And this sadly affects applications for jobs (a photo is pretty much required which would be considered illegal in other countries like the UK), apply for apartments (which country is your last name from... automatic rejection), just to mention a few key cases that really affect immigration.
i've lived+worked in 4 different countries on 3 continents and i think you always have to expect to adjust to the culture, it's not going to change for you, nor should it. But if you want to progress professionally (and Germany NEEDS tech-imports, the tech culture here is a disaster, it's embarrassing) you're going to have to promote these people into high positions, not just view them as "cheaper labour".
No - why do you do it? For upward mobility in a job market that is screaming out for foreign skilled workers.
When that upward mobility is no longer there, why should they stay?