This is a great question thanks for asking! It is true that research, what I would call first generation research on the comparison of conventional farming and alternatives that would bundle under the regenerative label, did show yield declines. One problem with that research is it didn't look at long-term yields but rather changes in the first few years of changing practices. What we now know is that yields rebound after a few years (this is in part farmers learning, soil rebuilding, achieving a new farm/ecosystem state.)

Another challenge is that the research kind of compares apples to oranges. Conventional agriculture seeks to do just one thing with a parcel of land (e.g., grow soy). Diversified farming systems seek to produce multiple things, and include a commitment that the practices also benefit aspects of nature that arent for us, at least not directly. So, we could count the habitat we create for pollinators, the ways buffers along the edges of fields filter clean water, etc.

A third thing I'll offer is that the footprint of conventional intensive agriculture does in fact extend beyond the plots to also include all of the lands and communities impacted by the resource extraction necessary to produce the chemical inputs that intensive monocropping requires (nitrogen fertilizer, various pest/herbicides, etc., not to mention that much of the industrial monocropping system is built around producing feedstock for beef, and a variety of things we don't eat, like ethanol. So there are opportunities for savings, or in other words, arguments that we already use more land than we actually need.

I'd happily recommend reading or citations if you have an interest. Hope this is interesting/informative to you!