Here the point is not to have an underclass of long-term, exploitable unemployed but to have some slack, ie. a minimum number of people available to ensure the system runs smoothly without shortages and hindrance to business growth and change.
Low unemployment is good and desirable but shortages are not.
If a society had a basic income for everybody, the unemployed wouldn't be poor, and would still be available for work if the demand for extra labour was high enough.
Although ultimately, I do think that eventually automation should lead to less people having to work. Or perhaps preferably: people having to work less; we should also get rid of the 40+ hour work week, so we can divide the available work more equally, while also making sure everybody has enough time to enjoy the fruits of their labour.
And part-time work also provides the needed slack in the system without condemning anyone to poverty.
The slack can be heirs forgoing the size of their profits, by boosting wages where needed, as opposed to slack in the wages if the people doing all the work and creating all the wealth.