Why is it bad to have 20% of the population involved in food production?
Why is it bad to be a peasant? Why is it so much better to be an assembly line worker?
Somebody: Yes, not only is it better to be an assembly line worker than a peasant, but it's better still to be a bioengineer, app developer, or particle physicist. Look, if we're aiming for a stagnant society, with wealth flat-lining, then sure Soviet-style regimes can work. The fact is, most people end up demanding more.
Why does it matter what percentage of the population is involved in agriculture? Because it's a fundamental indicator of agricultural productivity. Advanced countries can produce enough food to supply themselves with much less than 20% of the population. A lower-tier industrialized nation like South Korea has 7% of it's labor force involved in agriculture. Japan, which has encouraged small-scale cultivation since it's land reform, still has only 4% of it's population working in the fields. A country where one in five people cultivate the land isn't fully developed. Again, this only matters if we share the traditional Marxist goal of developing the productive forces.
I was looking at some health statistics for East Germany. The country has gone through real growing pains over the last twenty years and still suffers from high unemployment. Even so, just the first ten years after the transition, which we would presume would be the most difficult period for the country, still saw greater improvements in life expectancy than the previous decade under socialism. And whereas most of the improvement under the GDR was due to falling infant mortality, the greater degree of progress since reunification is because of falling infant *and* adult mortality. In other words, social democratic Germany has done a better job of providing advanced health care than state socialist East Germany.