It's not about the raw number of people, it's more about how old and aging they are. Age distribution is the key factor. 1 billion 60 year olds is not the same as 1 billion 20 year olds.
It should have no effect on fertilizer or phosphate prices, because it is a cheaper method to produce certain organic compounds of phosphorus.
Phosphate is used as an input for these chemical processes, but the fraction of the phosphate production that is used for such processes is very small in comparison with what goes into fertilizers.
The only possible effect is a price reduction for some organic compounds of phosphorus, e.g. for some pesticides.
I don't know about costs but phosphorus is a limited resource and more than 70 percent of the world's supply of phosphates is controlled by one country: Morocco.
Some estimates put "Peak Phosphorus" as early as 2030 but unsurprisingly, that claim is highly contested.
They are occupying the space left behind by human activity. Chemicals will only create more resistant strains just like it did with Amaranth. The reason these plants are successful is the horrendous landscaping traditions especially in new developments. The use of chemicals only reinforces these plants. Basically what's happening is these strains are put under so much pressure in the urban environment that they becomes "super-weeds" and then invade the rest of the landscape. It might be too late to do anything about it now.
I don’t think stinknet requires cleared land. Vegetation in the Arizona desert doesn’t form an unbroken canopy. Water is the limiting factor, and stinknet is apparently more efficient than any native plant at immediately converting water into explosive growth and immensely prodigious seed production.
It may well be that the natural course for stinknet is to dominate all other desert vegetation to such an extent that it forms a monoculture that fails to maintain ground nutrients and creates evolutionary pressure toward symbiosis with native plants. But it could take a hundred thousand years for Arizona stinknet to become a good citizen.