That's simply untrue. For example, the majority of people of European decent in the US, Canada & Australia are from the "peasantry" classes (ok, by the time mass emigration was happing the "peasantry" class had mostly migrated to the factory cities in Europe, but they were still the poor people).
In Australia's case the early white population was mostly prisoners who were almost entirely poor, unprivileged and most certainly not nobility.
Source?
IIRC, the genealogical records (of most present-day Europeans and Americans being descended from the nobility) was presented as fact, but the causal link with the rise of the middle class was more tenuous.
I'll look again after I've gotten some actual work for $realjob done.
I'm not totally convinced about the theory, but the book was a surprisingly good read.
That being said. What we call 'The Middle Class' in the United States is kind of deceiving. Class can't simply be a function of how much cash income you bring in a year. Class has more to do with where that income comes from.
Those of us trading our time in exchange for money are working class, even if ( in the rarer case ) it is a greater dollar amount from a person whose primary source of income is the buying low, and selling high of commodities... such as, human labor, or pork bellies... and accumulating and reinvesting the difference.
Not true for Britain. The peasantry did mostly die out but the nobility did very badly as well. The middle classes were the forefathers. Sources etc. in "Farewell to Alms" by Gregory Clark
Carlisle Cullen. ;-)
Anyway - this is a really good point, but I'll point out something else. Most of the wealthy would-be trillionaires whose fortunes got wiped out passed down another inheritance: their genes. Those same disasters that wipe out the savings of the rich tend to kill the poor, who simply can't afford transportation away from disaster areas, bribes for border guards and security forces, political connections to ensure safety for their children, or information savvy to know when a crisis is coming.
Most of today's middle class is descended from the kings and nobility of the high middle ages. The peasantry just mostly died out, leaving no descendants.