You got a few things the wrong way round.
First, the last few decades it wasn't the demand that was going down but the supply was going up with each generation joining the work force being larger than the one leaving into retirement, and also more women joining the work force.
These effects have ended. There aren't more women to join the workforce and the baby boomers, the largest generation that ever existed going into retirement.
Also, you are forgetting what governs the demand for workers. It's not some mystical fixed amount of work that needs to be done. A main feature of capitalism is that consummation is only governed by the available money, and it's practically limitless apart from that. If people have infinite money, they will just buy 10 cars. Not because they need them, but because they can.
That means if there's enough money around, there's virtually infinite work to do and thus infinite demand for labour. The demand is only bound by the amount of money people are able to spend.
This leads to the current crisis. It's not a crisis of too little demand for workers, but one of a bad economy. If the economy picks up, companies will start to hire again.
