Speaking as an eco-marxist, I don't think we do claim net loss or profit; capitalism isn't a balance sheet in any case but an evolving system whose benefits and disbenefits both grow over time; the problem is that its transitions (or those it inflicts on ecosystems for instance) can be discontinuous changes in equilibrium states -- changes of quantity producing qualitative, stepwise phase-shifts. A small net profit for humankind in one era may produce feedback effects which later doom the planet. For example, in 1966 less than 10 ice bergs drifted south as far as northern Scotland; last year over 1000 did. This is because the polar ice sheets (both of them) are breaking up. This stepwise change which may indeed produce a net loss for humankind, is the result of anthropogenic climate change, ie capitalism. And a left which does not begin its politics with these issues is just not worthy of the name.