In my opinion, what's happened has been quite toxic. People have been given the illusion of social progress whilst true opportunity has been concentrated in a very small population. Those that want their country back have missed the point: it was never theirs in the first place.
What this article refers to as "middle class" in Britain would probably be referred to as "working class".
To confusing things further, in Britain referring to something or someone as a bit "middle class" would mean it was somewhat posh.
Eg Rich lawyer or doctor? Middle class.
CEO of a FTSE 100? Middle class.
...and if you really want to go down the rabbit hole, when we say "public school" we mean a very middle class group of private schools, such as Eton or Harrow.
Middle class is more the common professionals - managers, small business owners, doctors, lawyers, accountants, engineers, architects, academics, creatives. The marker used to be a degree or some success running a business, but the degree part has been devalued now.
Working class is everyone else. Even if they work in an office.
Put even more crudely the upper class creams off the money, the middle class keeps the money machine running, and the working class does the grunt work.
Income doesn't correlate neatly with these groups. It's possible - not even as an outlier - to be middle class and working in the arts with very little money, or be working class and earning six figures.