Namely, that the amount of work that can be done has an upper bound.
In fact, the amount of work that can be be done is infinite and with higher productivity other things are done (= can be done economically) than before, for example:
Cleaning the streets, painting your house white, keeping your nearby river clean, build tanks & guns, maintaining bureaucracy, helping people you don't know (welfare state).
These things are all non-essential (yeah yeah I know but still ...), yet are only done now because only recently we achieved the productivity / wealth of being able to do it.
If we stop doing these things we will feel wealthier because individually, we'll have more resources to fulfill our needs. We would maybe still help our neighbours or paint our house white or buy guns but we would (or not) do it because we decided it ourselves.
I suppose in the future there will be other things we'll do that now seem insane: maybe even the poorest might have a swimming pool, or we commute a few thousand kilometers to work, or we each carry a wearable computer around without needing it for work (can you imagine???).
But there will be work, it is as certain as death and taxes.