I'd probably agree with much of that AV5 but I think viewing the political spectrum as simply left and right in that sense doesn't get to the heart of what is happening. When people are feeling affluent, things are going well, their sense of identity is not as important to them. When they are marginalised identity becomes much more important - you can see this in marginal groups of all types (right, left, anarchist, criminal, whatever), that their sense of identity is what keeps them going in adversity.
When it's possible for a demagogue like Trump to make a largely privileged group feel like they are marginal, or even when a large section of society is objectively marginalised, then it opens up the way for that group identity to become a major political force that can be exploited. Trump stooped pretty low to tell people they're being shat on by Muslims, Mexicans, free trade itself and anyone else he felt he could get away with offending. Objectively many of those people from the rust belt, etc, could rightly feel aggrieved as well.
Failing to recognise this is the same complacency that has seen the left fail elsewhere. I'd say accusations that the left is dominated by metropolitan elites who don't actually care about the working class voters they were once supposed to champion have substance, they certainly aren't addressing these problems seriously and if they continue to ignore divisions like this democracy as a whole is weakened.
The left no longer really have the option of reaching for the center because much of their base has been cut away or simply co-opted to their rivals. In the UK for example there is now a vast swathe of the working class that voted for Brexit that the Tory party, which would never have dreamed of gaining these votes in such numbers, is clearly attempting to parlay into permanent support with tough rhetoric, solidifying their grip on Parliament for a generation.
I think there is something more serious at work at the moment than the normal ebb and flow of party politics.