We could cut down on oxygen use by not breathing too. Ultimately you have to ground these abstract vectors of comparison in some fundamental purpose. For me, the only possible ground is humanity.
If humanity is the ground of your analysis, meat consumption will fail to present itself as the decisive factor for any vector of comparison. The decisive factor will generally be the elevation of capital or oligarchic social ends over general social interest.
It would allow nature to begin to recover from the damage we've been doing to it for thousands of years. Besides increased biodiversity being an inherently good thing, it would also reduce flooding and erosion, stabilise weather systems, preserve topsoil, etc. etc.
It's not just land use, it's every single vector of comparison, like water use, energy use, human labor (ab)use, habitat destruction, etc.