The problem we expected to have with a "modern economy" was, basically, not enough work.
And to some extent that's true. We will, eventually, pretty much replace/automate all the low end simple jobs, and even some of the medium complex ones.
Indeed, now that we have gone from 90%+ agricultural workers to ~1.3% agricultural workers, we have almost 89% unemployment.
There are three reasons a job will be automated instead of manual:
1) To Save Money
2) To Save Money, and
3) To Save Money
The fewer available jobs there are per laborer, the cheaper it is to hire labor, the more available jobs there are per laborer, the more expensive it is to hire labor.
We see this in companies taking factories/call centers to cheaper countries, we see this in low pay for entry level jobs(more available labor if it requires fewer skills), and we see this in highly paid jobs that require more experience, worse working conditions, or some combination thereof.
The only time this fails is when regulations artificially raise the minimum price of labor above the value of a job(as was seen when many low paid or entry level jobs went away after minimum wage laws made them too expensive for the value returned)
*IF* we could build robots today that could replace every single job, it would still take years or decades to build enough to replace the current work-force.
As it is, we have lots of development work still to do before we can get anywhere near that, and so it will be decades or centuries before all of the current jobs can be replaced(if they ever can be).
Between 'artisanal' crafts/goods/foods, e-sports, entertainment, and online 'influencers' how many completely new jobs do you think were created in the last couple of years? The last couple of decades?
Unless and until machine creativity and flexibility eclipses that of humans, there will always be jobs that are not suited for, or cannot be performed by automation.
If people are not smart enough to come up with their own new jobs, they can always go to work for the entrepreneurs that are, just like today.
And just like today, if there are 'unemployable' people, it is due to legal/political reasons, not economic ones(ie minimum wage and labor laws).