I've lived here for decades ifnot and I firmly disagree, for a start how far back do we go to "living here for generations" cause there was such a flux of people during the 1950s and the war that very, very few people could claim to have been here for an extremely long time. I live in a great community, but a community in a city that is so transient takes time to build and thats a difficult thing, you can't just sit where you are and expect to stay when everything else is changing.
There are loads of "up and coming" places, you just need to realise that the gains made by people in the last 20 years are unlikely to happen again, but that property won't really fall in value and give it a go. Harlesden, Colindale, Enfield, Tottenham, Palmers Green but if you mention this to people they turn up their noses. A complete lack of awareness that when I first moved here people did the same when Crouch End was mentioned, and absolutely no one wanted to live in Finsbury Park when I rented on the Stroud Green Road in the 1980s.
Also, I think you'd struggle to find areas of London that aren't mixed, my area mixed, the area I work in mixed, Westminster is mixed, Hackney is mixed, Brent, Enfield, Harringay are all mixed. Your exaggerating massively.
Living there for generations doesn't give you the right to stay, especially as most property in London is lease hold in the first place.