To add on to what others have said, the concept that capitalism is just letting the economy do its thing with no government influence is really mystifying and innaccurate. Capitalism requires immense support from a state (some sort of apparatus with a monopoly on force) in order to guarantee and enforce property rights, contracts, the collection of debts, ensure stable currencies that are widely accepted as payment etc. Just because the state is overwhelmingly working on the side of people with capital to preserve and accumulate that capital, doesn’t mean it isn’t working.
This worked (to some extent, in the small cohort of industrialized capitalist countries as a sort of class collaborationist regime mediated by unions and a relatively activist government) for around 20-30 years after WWII but that’s exactly what it is - something that will only work temporarily and for as long as it’s tolerable to capitalists, because the political system is built by and for capitalists, and as soon as they see an opening they will use the state to beat back and discipline labor (in this case the neoliberal reaction that’s continued since the 80s). Reformism is a circular dead end because politics and economics are inseparable, and political power just like economic power under capitalism is always (in the long term) gonna be stacked in favor of the people with capital - and those people aren’t gonna give up their power without a fight.
That analysis is also looking at the whole labor market as a closed system within rich capitalist countries when the reality is that most of the breathing room that the middle class / unionized labor had during that period was built on top of capitalist super exploitation of labor in Africa, South America and Asia, and that sort of exported exploitation is always gonna be the case under a capitalist political system built around nation states.