Speaking from the outside here, but it's always seemed ridiculous to expect complete and total assimilation.
Maybe it's the word itself, which reminds me of a hive mind or cult.
I've just never really found the version of integration brought up in these conversations as fair. The idea of a sub-community being formed that's integrated and accustomed to the culture of the host country? Yeah, that's appealing. But the way I see it described is as if immigrants should drop everything from their past and their culture to act just like the other residents of their new residence.
Ideally, integration should involve a degree of cultural exchange: the Indian-American community has many of its own unique attributes that distinguish it, but members going to India aren't suddenly up to date with everything there (this was a random example, I'm not sure how good it is). But conversations that I see come up seem to defend this pre-set notion of what being German or Dutch or Swedish or French should mean, when ideally that should be fluid. Tbh, I don't care for whatever pre-set definition of these cultures is.
The opposite of "assimilation" seems really dumb too. "Leave them alone" types of approaches, or putting immigrants in isolated communities, both seems extremely naive and lazy.
Integration should happen, but it will happen in a more welcoming and accepting environment that tries to mix things up. The existence of sub-cultures within a culture shouldn't really be frowned upon for not being like the rest of the culture. If anything, those sub-cultures can add variety and appeal to culture as a whole. Same with the act of new immigrants joining or moving into similar communities (newsflash: when you're in a strange new country you're going to be living in, you want a degree of familiarity in your surroundings both for practical purposes and homesickness). Ideally, if your country hasn't fucked up the integration and available interactions for these smaller communities, other members can act as a springboard into interacting more with the host country's culture.
But if the communities themselves are already isolated and withdrawn, then the solution needs to focus on fixing that, not trying to remove these communities entirely. That means not alienating them and not creating laws or an atmosphere that decreases the chances of interactions outside the community.
tl;dr
A. Assimilation is a terrible word to use there
B. Integration means the immigrants should get to keep or mix their previous culture, and the host culture takes some of the new one. It doesn't mean everyone that moved to France should suddenly act according to some definition of the french.
C. Sub-communities are good and can act as springboards for integration, but only if they're not isolated
D. Every time an isolated or discriminated community is alienated, it will just cause said community to be more withdrawn, and decreases chances of external interaction and integration.
EDIT: And no, cultural exchange doesn't and won't (for the paranoid ones of you out there) mean that everyone will be out wearing burqas and oppressing women, nor that that should be accepted. But if you're gonna act like everything in these immigrant cultures is reprehensible or backwards, that maybe voting far-right makes sense for you