I think social mobility is one of best measures of a successful capitalist system, and social mobility has been declining in the United States since the birth of neoliberalism.
People think that wealth is the core conceipt of capitalism, but wealth and power have been a part of every system before it. It's competition for capital that is the great innovation of our system.
Healthy capitalism is about maintaining the balance that maximizes marketplace competition. These aren't black and white questions, they are fundamentally a question of balance. This is why we have anti-trust, this is why we have the regulatory state. When you remove these things (or worse, hand them to billionaires to weaponize them), capitalism crumbles, and becomes oligarchy.