I think calling "today's billionaires the early adopters of future living standards" does incorporate two debatable assumptions, but they're implicit assumptions rather than the explicit one CellBioGuy called out. The implicit assumptions are that (1) future economic growth translates into real income growth throughout the income distribution, and (2) being rich in the future will actually allow one to mimic the lifestyle of today's billionaires.

(1) could be false if economic growth just ends up being captured by e.g. the top 10% of the income distribution. If so, decades from now most people's living standards might remain below those of today's billionaires.

(2) could be false if components of current billionaires' lifestyles become difficult to buy in the future. For instance, a billionaire today can buy pretty well any dwelling they like. But even in the future I doubt most people at the 20th percentile of the income distribution could do the same.