I don't want to reveal any information about my personal life. However my post does give away some clues about this. I understand that I might not be able to make convincing anecdotal arguments without revealing this fact. Unfortunately however, I predict people are either going to immediately agree with my post, or never believe. I don't think there's any point in this discussion other than the joy of writing.
> They consider themselves lucky if their grandkids still speak their "native" tongue at a primary school level
Not all populations approach assimilating into the host society in the same manner. For the purposes of my argument I'll limit the scope of the term 'assimilation' to 'Adoption of the host country's core values'. In my city large large immigrant enclaves have developed over the last few decades to the exclusion of even the host country's culture. You could walk through large suburbs in my city and believe you were in a very cosmopolitan area of Bangladesh or Pakistan. Anecdotally, many sources report that the second generation of immigrants from the Middle East have become more conservative than the first. Adopting much a much more conservative form of Islam, which many view as fundamentally incompatible with Western democracy.
Analysis of available statistics from a recent survey on support of same-sex marriage revealed that immigrant communities were among the least likely to support gay rights. This is congruent with the commonly held view that women's rights and LGBT rights do not enjoy a similar level of support within the Islamic diaspora to that of wider Western society. I could go on...
Multiculturalism is fine at the scale of a football team. The rules of football are so simple they can be communicated non-verbally. However, extrapolate the problem to the scale of organising society and you'll find that people have fundamental disagreements that simply fail to be reconciled.
> What I have seen everywhere, however, is a lot of small minded people who are... My apologies if I've got the wrong impression..."
To me, this subtle ad-hominem undermines your post. To me this simply reads as "People who disagree with my world view are small minded." I have made no such insult towards you. You haven't done anything to actually challenge my ( omitted ) definition of 'assimilation', save to assert that my disagreeing with your overall views reveals that it must be "flawed" somehow.