Can you point me to a dictionary that specifies, that it can only refer to the current conditions?

https://en.wiktionary.org/wiki/sustain

It suggest as meanings to maintain, to provide, to encourage... In the meaning of provide and maintain there is no limit to current conditions.

I have laid out the conditions under which the world can sustain such a human population. I find it linguistically wrong to limit it in such a way, that only the current situation is permissable. This is directly contradictorary to any use in relation to future like planning.

E.g. "we plan the building to sustain a 6.5 earthquake" would be wrong under your criteria, as neither the building, nor the earthquake exist at the point of that statement..