I know the difference between herbicides and microRNA but you do not seem to see the assumptions that you make.
At some point, you take calculated risks, because there is no action in life that comes without risk. Even your preferred solution of eliminating all chemicals and switching to alternative farming practices comes with substantial risk: it’s very likely it won’t scale. Millions would starve.
Organisms may be able to handle everyday quantities of dsRNA, but beyond a certain concentration, you may suddenly see unexpected problems, which then cause domino effects throughout the ecosystem. These kinds of relationships are unknown unknowns, and they rarely behave linearly.
I really like the RNA-based approach, don't get me wrong, but the other commenters here are right to point out that hubris is dangerous.