So I would be interested to know would happen here.
For far lower returns. With reduced demand, there's not much for that extra money to do. To the extent investment drives growth it's when it goes into innovation (R&D) or if businesses have demand but are capital constrained (and I suspect even that's more like speeding up than creating growth, exactly—you just get 5% growth now instead of 2.5% over two years, or whatever). Sure maybe you could spend it on advertising and marketing but all that can do is get us back to "people spend too much".
Don't be so sure.
Savings rates have been inching up ever since bottoming out around 2005.
Household debt (latest): https://data.oecd.org/chart/5VU6
You can play with the charts to plot trends for as long as the OECD have data for each value. The position of the US isn't great, but it isn't terrible either.
That's why I find the idea of this system collapsing frankly quite amusing. Actually I hope for a complete break down of our economy. It's a perverted monster which deserves to go down.