I'm comparing it by ... comparing the nutrients in each. There are lots of examples of nutrients in meat that aren't found in plants (carnitine, b12 etc) and even more where the type found in plants must be consumed in far greater quantities and converted to a form we can use (retinol vs beta-carotene, DHA vs EPA/ALA etc)
Millions of vegetarians doesn't mean meat is suddenly less nutritious.
The B12 in off the shelf beef , chicken is from it being injected by farmers (along with all manner of other stuff such as antibiotics).
The truth is that every living being is low on B12 due to soil erosion / over farming. So in a way a meat eater is supplementing B12 by proxy of an animal, where as a vegan is buying a pot off amazon (and in turn able to get a much more specific dose).
We all live on this planet together, what we do affects others and while I think meat should remain part of our diets I think it needs to become a much less central part of it.
I enjoy meat myself, but I use it more sparingly in combination with other central meal components.
Millions of vegetarians doesn't mean meat is suddenly less nutritious.