Rebellion and dissension used to be the hallmark of the young
I think that's the problem. The baby boomer generation rebelled and did radical things, and they did it better; free love, polyamorous communes, drugs, black power, androgeny, socialism, communism, anti-apartheid, anti-nuclear, radical feminism, live aid, everything.
Then they ironically settled down, brought property, became conformist, had 2.4 kids, a dog, a volkswagon a range rover, and a house or bungalow and a job for life and now live in large part conservatively even if they are liberal to a degree.
Meanwhile subsequent generations picked up on those ideas but without the same degree of critical appraisal. It's all 'don't conform' whilst also doing don't question, underpinned by a hyper-individual mindset.
All well and good, but when you criticise and reject all the tenents and instituions that serve to bind as a national identity, then yes there's a loss of national cohesion.
Someone upthread decried religion. All great civilizations have needed religion with ensuing rites, and passages and observances. It binds people and thus nations together. You cannot dismantle it without ensuing chaos or giving power wholesale to the state.
People now, replace religion with causes and become dogmatic about them, showing intolerance for other peoples point of view ramming their causes down others throats.
Religion can also be a check on political power and on the individual. It's a balance one that's been done well in the west until now.
The problem is immigrants are not letting go of their traditions or their religions and appear to have a stronger cultural identity. But it's not their fault that countries like the UK have turned away from theirs.