Hola AA

If you interpret this as you said "for the benefit of future generations" you would still be using "para" as it is expressing "the recipient of that action". I can't see any possible interpretation that triggers "por" instead, unless you want to convey a "cause" as in "preserving natural resources because of future generations", maybe interpreting this as [something like]: the act of preserving natural resources should be done because of the new generations, because of them, as in they are the ones for whom this would make sense as they will be the ones on the planet. (if you know what I mean...) But, it wouldn't be a natural thing to say unless there is another verb that justifies a bit more that "por", like "hacer algo por las futuras generaciones".

Saludos