People have had to deal with desperate, impoverished lives throughout all of history, and usually managed to muddle through without committing suicide en masse or dying of drug addiction/overdose. People in the past most likely managed to deal with their poor circumstances because of social factors (i.e., they weren't lonely). Throw in an epidemic of loneliness and social isolation, as is common in modern urban/suburban life, and it's easy for even people in not-so-horrible circumstances to break under the strain.
Indeed. It should be clear that measuring loneliness is quite difficult given measuring any feeling is hard and given people may define "not lonely" relative to whatever they've experienced as that rather than some absolute criteria.
Certainly, one might attribute the growing epidemic of opioid addiction and suicide to loneliness - or to despair but it seems reasonable to attribute it to something like that.
And there's no reason loneliness, inequality and etc aren't connected problems. Once, one could point to those with lower incomes still having the vibrant social life of "the streets" but the poverty of those in outer suburbs does not necessarily generate that same situation.