That’s not the logic at all.

The logic is what I said. If you start growing more plants it will not fix monocropping issue. It will only make it worse.

If you don’t want to eat beef that has been fed monocropped feed then vote with your wallet and buy sustainable meat that was pasture raised (on non-arable land).

You can't just claim that switching from corn and soy to a wide range of other vegetables would make things worse with no argument. If nothing else this goes against basic common sense because (1) we eat more than two vegetables, and (2) people eating vegetables directly instead of feeding those vegetables to animals is a vastly more efficient way to feed a population.

The absolute best case scenario for your argument is that monocropping turns out to be just as bad under a shift-to-vegetables model, but in that case we still end up reaping enormous environmental wins. Just because vegetables might not (?) solve the monocropping problem doesn't mean that we don't have good reasons to switch to them, as was originally argued. I'm not really sure how you avoid that, and frankly, the number of large scale climate change studies urging a shift to our diets seems to bode poorly for attempts to push this line.

It's not remotely scalable to convert the entire beef supply to pasture raised. With that out of the way, and with an understanding that monocrop fed beef uses a lot more land that monocrop plants per calorie, assuming that people won't eat more calories on plant based diets vs meat based diets, it's simple math that plants will reduce monocropping. I don't get how you can't see this.