While it's true that new inventions do make people's lives materially better, the point that's being missed is... this is an inevitable consequence of the development of society, we'd have these new inventions regardless of our economic systems or choices of where to tax people, or economic policy. Those benefits aren't caused by the widening wealth gap, and the wealth of billionaires isn't required for new inventions to be made.

What we're talking about here, specifically, is that people are producing more wealth, but not getting wealthier. The wealth of people has "effectively stayed constant since the 1980s" but the total wealth we're creating has gone up significantly. It's certainly nice that we have more things we can spend that wealth on, but that's a distraction from the issue of where the rest of the wealth is going.

If you actually look at people's lives, "materially" and look past the inevitable march of technology, what else is happening? 70% of people are "living paycheck to paycheck"; poverty, and child poverty are massively increasing; many schools can't afford to give children textbooks; and teachers have to buy school supplies out of their own meager paychecks; the cost of healthcare is increasing, leaving many unable to afford it (or in non-USA countries, where everyone can afford healthcare, waiting lists are increasing due to poor funding); Towns are bankrupt and can't afford to repair infrastructure; the quality of most goods (clothes, houseware, and furniture especially) are decreasing to the point where they fall apart in a few years; and most public services are on the verge of collapse.

The cost of housing is so high that younger generations don't even aspire to own a home any more, the cost of higher education is so high that people expect their student debts to never be paid.

So yes, it's nice that we have Facetime and Playstations, Those things do make our lives better... but you have to consider, if the "increased productivity" of workers was going towards society and making our lives better instead of enriching billionaires, could we address some of this?