I have a dear friend I've known for 30+ years. She's French. She's lived in the UK since her 20s, married a British man (now divorced), they had 2 kids who are British. She never intends to live in France again, she doesn't have good memories of it.
After Brexit, she was asked to fill in a form, she can become a British citizen just by doing that, she has all legal rights to stay here. But she's refusing purely because she's annoyed at having to do it at all, she doesn't think she should need citizenship. She'll either fill the form in eventually or get it automatically on retirement but I genuinely don't understand her attitude.
She wouldn't have to give up her French citizenship and she has benefited de facto from UK citizenship for many years. If I were in her position, I would absolutely expect to make the commitment of taking out French citizenship alongside my British citizenship. I've had to really zip my mouth on this issue with my friend.
She's a first generation migrant, it's still less understandable that second generation migrants would not want citizenship of their country because of a cultural allegiance to their parents' country unless the parents never intended to stay and everyone always intended relocating to the original country.
I lived and worked in Scotland during the time of the independence referendum, so would have been entitled to Scottish citizenship had the vote been yes. I am English and I didn't support Scottish independence for a number of reasons. But had the vote been yes, I would have taken Scottish citizenship alongside my British citizenship because I would have been living and working in a nation who had made that decision, and I would have respected it.
Citizenship is a privilege, bringing rights and responsibilities and generations of migrants have made the commitment to become British, even where they have been discouraged or had an unknown future ahead of them and I hugely respect that.
I didn't vote for Brexit, but I think the EU system seems to have undermined the idea of national citizenship with a belief in borderless movement, picking and choosing to suit individuals.
Of course you should make the material commitment to become a British citizen unless your heritage identity means more, in which case, you will need to live with uncertainty.