Monocropping is vastly more efficient on a number of counts, especially labour input. I can’t emphasise enough what a big deal that is, it’s really the only reason anyone is getting fed.
Yes, there’s a lot of problems with agriculture - predominantly its scale and extent, but eight billion mouths take a lot of feeding. Topsoil loss should be our major concern, together with biogeochemical losses. Every other issue is fairly minor in comparison to those, really.