I understand this issue of maintaining motivation under conditions of artificial full employment was a problem under the Soviet system, but I'm not sure it is necessarily a problem in a post-scarcity economy. For one thing employment can be increased by reducing the work week. After all we went from 60 hour work weeks to 40 hour, even less in Europe where there are longer vacations than in the US. Education over the past century has gone from mostly primary only to mostly college, absorbing excess labor capacity.

I think the idea of 80% leeches living off the 20% productive is just wrong headed. What we will see is the general quality of life improved for everyone. Sure there will some that need to be entirely supported by the rest of us: the homeless, the mentally ill, the infirm, the elderly. The real danger is that the increases in productivity are being siphoned off by the rich and not being equitably distributed. Capitalism is inherently an unstable system tending towards monopoly and the destruction of the commons (pollution and the like) and must be reasonably constrained.