When I said "relatively new" I meant "in the past 300 years or so". When I said "any old philosophy book" I meant "one written 1500 years ago". I didn't make my frame of reference clear; that was obviously a mistake on my part. Saying things like "new" on a tech forum has a very different meaning compared to "new" in terms of history and philosophy. 300 years ago people were born in to a strata of society and couldn't work their way out even if they wanted to. Social mobility in the middle ages didn't exist (with a few notable exceptions like demonstrating valour in warfare.)
More recently (in the past 50 years) there have been some quite extensive studies of happiness, and envy is almost always found to be a confounding factor. Why that is seems to be the case is an interesting evolutionary question - https://www.frontiersin.org/articles/10.3389/fpsyg.2017.0161...
A philosophy book offers conjecture not evidence.
Such a speculative statement. You cannot predict the behavior of a group of people anymore than you can predict a stock. I would say a more realistic statement to your topic is:
People can be happy with what they have and be unhappy with what they have and throughout history many people have felt either way. It is not a new phenomenon. Whether these feelings lead to revolution in the future is impossible to determine.
You need to look at trendlines, data points and evidence.