The current population of human beings on this planet is unsustainable.
If that were true, then wouldn't the population be decreasing instead of increasing? It's like saying you are in a plane and you slow down to below stall speed and say the lift generated by the wings cannot sustain the weight of the plane yet the plane continues to fly. Until populations decrease, all the evidence shows that the population is sustainable.
It's been a while since my college biology course, but if I recall correctly, it's not uncommon for a population experiencing exponential growth to shoot right past the carrying capacity for a while. Eventually, the population crashes until numbers fall under the carrying capacity, at which point several outcomes are possible.
The simple fact that population size is not decreasing is NOT evidence that the population size is sustainable. Rather, the fact that population size isn't slowing down might very well be evidence that it ISN'T sustainable...