Might just be me but I'm missing the relevance. The point is that a household which has already maxed out its possible number of workers no longer has the optio

Might just be me but I'm missing the relevance. The point is that a household which has already maxed out its possible number of workers no longer has the option of sending someone else to work to deal with a sudden expense, so a two worker household is more fragile than a single worker household at the same income. The only reason I bring up productivity is to mention that it hasn't come down in proportion, which this graph doesn't show either.