The cumulative inflation for the past 10 years is 35.6%, not 7x as you imply.
Without taking a stand on which metric is better for social and lifestyle comparisons, the grandparent poster said 'valuation of assets' rather than inflation. Per Yahoo Finance (https://finance.yahoo.com/quote/%5ESP500TR/), the S&P500 is about 340% of its mid-2015 level on a total return basis (reinvesting dividends).
Today's $700-millionaire would have been a 'mere' $200-millionaire in mid-2015 if invested fully in equities. That allocation is probably ballpark reasonable, since with that kind of net worth investment horizons are very long (multiple generations of inheritors) and the investors themselves can be nearly risk-neutral rather than risk-averse.
I appreciate the correction, but that's still nowhere near the 7x that the OP was talking about.