It's not as much a disaster as you seem to think.

Surely, you'd have a lot of people who are not in the active workforce. But due to automation + AI, we're going to see a mass layoff crisis in the next 10 years anyway, so if people retire, that's a good thing that young people can find workplaces.

Also it should be noted that large swaths of the population didn't participate in the economy back in the 19th century, when 1/3 of people was below 15 years of age. Still the world didn't end because of that, the opposite, that's when growth really set it. The lesson to learn here is that a large active workforce is not required to have a good economy.