We have labour-saving technology though.

Automation leads to higher productivity, so those fewer people can earn more and spend more.

The economy will do just fine.

I think it'll be a rocky time though, we're facing a new Industrial Revolution - where the existing capitalists would prefer to drive down wages and destroy trade unions via mass immigration, and ordinary workers want to reverse their worsening standard of living. Brexit was just the start of this, but I think we'll see more radical movements until this is resolved.