Our society has become massively automated compared to the middle ages. And we have 25 times the world population now. Yet we still have plenty of jobs;
No we don't. It's be decades since any western country had full employment, or even a policy to achieve same, thanks to the sadistic neoliberal idea of NAIRU. In most of the western world, there are an order of magnitude more job seekers than there are jobs.
And that's not even taking into consideration the swathes of the population involved in unproductive, pointless, bullshit jobs that serve no real purpose (eg: most layers of management).
Within a generation, two at the outside, the vast, vast majority of jobs involving manual labour will be performed by robots, except for those targeting the high-end luxury market. I expect a fairly large chunk of today's "intellectual" jobs will also disappear towards the end of that timeframe (eg: basic engineering, software development, lower levels of management, etc) as AI capabilities improve.