I'm sorry to hear about your experience and as an American who lived in a few European cities I know what you mean, some neighborhoods are decidedly un-European, but I would partially blame that not on the immigrants but on policies that bring in so many at once. Of course there is no need to assimilate when there is a huge neighborhood of people just like you... imagine the driving culture if you brought a million Cairo cab drivers to Singapore? But if you bring 50 a year, they assimilate. That said, I'm pretty sure most economists (other than Borjas perhaps, whose seminal work on the Mariel boatlift seems critically flawed) would broadly disagree with you about the net economic benefits being negative and I would love if you could share that paper.

https://www.economist.com/the-world-if/2017/07/13/a-world-of...

https://www.cgdev.org/blog/what-mariel-boatlift-cuban-refuge...