There is no such thing in general as "too many people for the economy".
"The economy" is the sum of goods and services produced and consumed. Everyone who lives consumes, and could easily consume more. The economy is generally limited by the ability to produce, and is generally limited by the number of working-age people to do the producing. Unemployment can be result of production being bottlenecked by something else:
* raw materials - but Japan never had an abundance of those even when their economy looked like it would dominate the world in the 80s
* education - but the people the article talks about have university degrees
* capital - again not something Japan lacks
So it's not one of those. I submit that it's organization - the Japanese society and economy is extremely rigid and maintains some egregious inefficiencies in the labor market, simply because it's always been done that way.Companies would rather hire a new 22 year old graduate at full pay than give a 30 year old with an even better degree a chance at a 30% discount, not out of concerns over the 30 year old's skills being outdated but simply because hiring new graduates is what you do. They'll hire non-graduates only on temporary contracts when short of workers, but never in a million years consider promoting them and converting them to regular employees, no matter how good they are.