This is not the reason though.

The reason is that now there are fewer different kinds of jobs (many roles like manufacturing have moved offshore) and also fewer jobs in terms of numbers and more regulations which prevent competition and shrink the number of competing company (again leading to fewer roles).

It wasn't nepotism before, it was efficiency. I didn't have any connections and I got better opportunities back then than I do now and the people were overall better at their jobs. Nepotism is much stronger now and jobs are increasingly theatrical/bureaucratic and have little to do value creation.

It's possible that people's lifestyle in the west was propped up by the exploitation of foreign labour, that much is possible, but note that the people who did the exploitation back then are the same doing it now (or their heirs). The difference is that now the money is increasingly circulating just between each other, bypassing normal people. Soon it will bypass all humans; the entire economy will be between machine owners. Social mobility is mostly about social connections and politics nowadays. In the west, it has never been less meritocratic.