Assuming that we can maintain the current upward trend in productivity presupposes that we have limitless resources. We do not.
This statement is often made to alarm and only does so because of the assumption that the world is somewhere near the end of its resources. This specious alarmist argument has been made for over a century and the future has always not only proven it false but shown the reverse to be true.
Your other statement (with the examples of "increasing salinity of irrigated farmland, depletion of aquifers") again presupposes that these are problems that either will be insurmountable or cause some great disharmony or sacrifice before they are solved, and usually are tagged with the only legitimate solution being population control. Again, you should show some humility in the face of history.
Accounting for the world's land surface area, the amount of arable land, water supplies, increasing urbanization of populations, renewable land and ocean resources, and conservative projections in existing technologies and resource discoveries it is hard to say that the world cannot comfortably sustain 100 or even 200 billion people.
In light of the painless altering of fecundity seen in developed nations, as well as a knowledge of basic ecological growth models (and the, albeit weak, evidence in this UN report), it is hard to say that the world's population must even attain such levels or that it would take enormous sacrifice or require conscious intervention to prevent it.
In short, the whole population control cult is dependent upon not just worst-case scenarios but extreme myopia to sustain its arguments, and taps into misguided fear and the human desire to control others for its popular support.