That sounds like article of faith. The GP is speaking about the UK - wasn't it always a very socially stratified society? What about other poorer European countries? In a cross-border comparison, there is no obvious strong correlation between the parameters you mention and violent crime, how could there be a causal relationship then?
And if we limit ourselves to the US, what about crime development in time? It certainly didn't rise and fall in lockstep with poverty, income and wealth inequality, not even in the last decades.
My grandpa grew up without electricity in his home, seven people in a single wooden hut. He was always a kind old man, even though a bit of a drunkard.
Poverty does tend to correlate to crime, but poverty never exists in a vacuum. Population density probably has a greater bearing on crime, and poverty and population tend to go hand in hand in metropolitan areas.
If you have an area that is poor but has few people, there'll probably be less crime, as less people means it's easier to track each person socially, and thus there's more accountability.
I'm sure there are studies that show optimal 'tribe' sizes, and I know from experience how different orgs can be from small teams to large teams, so it stands to reason that the key driver is probably population density.
we have to differentiate between 'low level' crime and 'high level' crime, e.g. fraud, financial crime, all of those super-rich organised crime
From WW1 until about 1975, no it was getting less stratified. The upper classes famously had to relinquish their country houses because they couldn't afford to maintain them and the middle classes grew.
It wasn't until neoliberalism took a foothold under Thatcher that social stratification started to grow again.
It got way worse during the Thatcher years and especially following them, with the formerly mighty heavy industry all but collapsing, and the Brexiteers made heavy bank on that. The aftershock of first Covid and then Ukraine with the gas price and general inflation was yet another heavy blow.
Knifings are still pretty rare here. I have met and lived with plenty of poor people and mere material precariousness won't remake one into a callous psychopath who behaves like a violent predator.
Drugs will, though. Frying the brain, as they say.
The approach that does: https://www.svru.co.uk/ / https://en.wikipedia.org/wiki/Violence_Reduction_Unit
You need both carrot and stick, and you need the police to target people who are doing crime and not target people who are not doing crime.
Media distortion driving fear is a whole other category of problem.
If we go out to rural areas, we'll probably find more neighbourly behaviour than we will in inner cities. Poverty, population density, economic status, and more. These all play a factor.
The problem at the core is that we rapidly went from high-trust societies where you could literally leave your door unlocked or even open so the cat could have free dominance over the local bird population and nothing would happen to a low/zero-trust society where you have to fear even a locked door isn't a guarantee that some crackhead didn't break in to get something to pawn for their next hit.
And to be very clear: more police, deporting migrants or whatnot else that conservatives/far right reliably preach as a "solution" will not help, because the root cause is widespread poverty and income and wealth inequality, nothing else.