Oh come on. We do in fact talk about the biomass of human populations, relative to what an ecosystem can sustain.
Broadly speaking, different ecosystems can support different total biomasses, as a function of total energy input into the system (mainly from the sun), and a variety of limiting factors (like availability of water). So a desert--which can be a perfectly healthy ecosystem--sustains, both in theory and practice--far less biomass than, say, a tropical rain forest.
Let's take an example, pulled more or less randomly from Google Scholar, an article titled "Hunting pressure modulates the composition and size structure of terrestrial and arboreal vertebrates in Amazonian forests". Here is the abstract:
>Overhunting is a leading contemporary driver of tropical forest wildlife loss. The absence or extremely low densities of large-bodied vertebrates disrupts plant-animal mutualisms and consequently degrades key ecosystem services. Understanding patterns of defaunation is therefore crucial given that most tropical forests worldwide are now “half-empty”. Here we investigate changes in vertebrate community composition and size structure along a gradient of marked anthropogenic hunting pressure in the Médio Juruá region of western Brazilian Amazonia. Using a novel camera trapping grid design deployed both in the understorey and the forest canopy, we estimated the aggregate biomass of several functional groups of terrestrial and arboreal species at 28 sites along the hunting gradient. Generalized linear models (GLMs) identified hunting pressure as the most important driver of aggregate biomass for game, terrestrial, and arboreal species, as well as nocturnal rodents, frugivores, and granivores. Local hunting pressure affected vertebrate community structure as shown by both GLM and ordination analyses. The size structure of vertebrate fauna changed in heavily hunted areas due to population declines in large-bodied species and apparent compensatory increases in nocturnal rodents. Our study shows markedly altered vertebrate community structure even in remote but heavily settled areas of continuous primary forest. Depletion of frugivore and granivore populations, and concomitant density-compensation by seed predators, likely affect forest regeneration in persistently overhunted tropical forests. These findings contribute to a better understanding of how cascading effects induced by historical defaunation operate, informing wildlife management policy in tropical peri-urban, rural and wilderness areas.
Let's think about this for a bit. You are making observations about lots of different animals. Some animals are common. Some are not. Some animals are small. Some are not. But you aren't interested in, say, just the effect of hunting on some particular species of woodland mouse, but across groups of species (e.g. frugivores, granivores, etc.), and as a whole. So, how are you going to do it. You found a 10,000,000 really healthy ants, and 5000 mice, 100 snakes, 10 weasels and 1 cougar. You can't just add them up. Sure, you could measure the decline in number of individuals from each of those in percentage terms, and then, maybe, what, average that? But aren't you really making a mistake doing that? What if that 1 cougar died, roughly the weight of all those ants put together. Aren't we missing something here? Yes, we are. And that thing is the total mass.
A fact like, humans and our livestock, account for (some huge proportion) of the total biomass in a particular ecosystem, that's actually pretty damn relevant, and meaningful. Less meaningful perhaps than comparing to the theoretical capacity it could achieve, if such numbers could actually be derived, but it still says a lot about the state of ecosystems today.