Regardless of whether Glyphosate is toxic to humans or not, it kills the rhizome (fungi and other ecology around the roots), drying the soil and making it less productive. Robots will not only avoid this problem but support the planting of complementary crops.
Without this job I foresee several potential problems: large unemployment, people leaving the countryside, many low paying jobs that feel meaningless to the workers.
It would make food expensive again. As of 2010, 1.6% of American and 14% of the world's workers worked in agriculture [1]. As recently as 1870, that figure for the United States was almost 50% [2]. Most of us prefer today's standard of living.
[1] http://data.worldbank.org/indicator/SL.AGR.EMPL.ZS
[2] https://www.bls.gov/opub/mlr/1981/11/art2full.pdf page 12
Yes. Expensive compared to other things.
> Most of us prefer today's standard of living.
Yet we've never been more depressed as a population...
Manual farm labour is extremely hard work, "stoop" labour causes back conditions, and man+machine agricultural labour has a history of accidents.
People already have left the countryside in the West; farming is a few % of the population, plus seasonal immigrant/guest workers.
I'm quite happy to be on this side of the agricultural revolution and increase the automation in farming. If we want meaningful jobs, we have to address the question of "meaning" directly, in the context of the society we wish to have.
Now if only the wage gap between the two jobs was not that big, I think many more people would realize this and be willing to do the hard labour of working the fields.
You'll get your answer by working in the fields for a few weeks.