I do not believe in the Marxist interpretation that capitalism will ruin itself. Even if we are doomed to have leisure at the end of capitalism, the machine that brings the leisure has to be maintained by someone. That means a reduction in work hours on the individual level.

A works 40 hours and demands 30 hours

B works 40 hours and demands 20 hours

C works 0 hours and demands 30 hours

Total labor supplied = 80 hours = total labor demanded

We call C lazy even though the technology we invent and work we do has the goal of reducing the need for future labor. The problem isn't that robots will make us unemployed, it's that we measure our prosperity through employment rather than the work being done. People will hoard employment opportunities and work more than they need to. This creates a intensifying race to the bottom as being 1% better gets you the whole job, rather than let you have a fair portion of the job market according to your ability and how much labor you yourself demand.

Let's say everyone works full time including C. That would mean 40 hours of meaningless employment. If the meaninglessly worked for surplus is then being invested then interest rates must necessarily become negative. Work becomes leisure and investment becomes consumption. This is the starting line of leisure capitalism.

Conventional economists would just say "if there is a surplus just lower prices (wages)". Yes, that is why wages are stagnating. However, what is the purpose of the wage stagnation? Of course, lower prices are a signal to stop production and to stop working. The employers' response is to fire whole employees rather than cut their working hours. That is why C is unemployed.

Of course there is a problem with this model. It assumes a perfectly flexible debt market. The economy as a whole is doing fine because C can just borrow 30 hours of work for 0% interest.
What if it isn't? If C cannot borrow because of his unemployment, then C will also cut his demand for labor to zero because his demand for work is capped by the amount of work he supplies.
This is why depressions and recessions are so ugly. Total demand is now 50 hours, B gets fired, total demand is now 30 hours. It's also why Keynesian stimulus is so effective. All it does is increase the flexibility of the debt market because governments borrow money with good conditions and give it to those with bad financing conditions via unemployment benefits or food stamps.

If A worked 10 hours less, B worked 20 hours less and C worked 30 hours more there wouldn't be any intense competition for the scarce resource of employment, there also wouldn't be any need for debt growth or endless GDP growth.

An oversimplified conclusion of this argument is that economics is effectively built around a pyramid scheme of population growth because children demand labor without supplying labor, giving the illusion that full time employment is the normal state of an economy and that labor can be stored through money and even earn a return because it's always replaced with more young labor.

Think about it this way. If I work a constant amount of hours but my productivity keeps going up and my personal desire to consume does not, then the only way for me to work full time is by having more and more children as the robots get more and more efficient.