If water vapor is always blown away into space, how is it able to create chemical compounds on Venus? This is the follow-up of this chem.SE question.

According to Wikipedia, water vapor on Venus is present in trace amount (20 ppm). There are multiple reasons why Venus has very low water content:

The rich carbon dioxide atmosphere generates the strongest runaway greenhouse effect in the Solar System reaching temperatures at least 735 K, hot enough to boil most of the water vapors.
Free hydrogen and oxygen atoms  has been swept into interplanetary
space by the solar wind because of the lack of a planetary magnetic
field
Water vapors are continuously blown away by the solar wind through the induced magnetotail.
UV and photodissociation of water vapor creating hydrogen and oxygen atoms and radicals
High D/H ratio in Venus due to hydrogen atoms escaping into space since it is light thus unable to form water
The atmosphere is rich in sulfuric acid clouds which is a powerful dehydrating agents. Most of the water reacts with H2SO4 to form hydronium (H3O+) and bisulfate (HSO4-) ions. As a result, the concentrations of “free” H2O in the acid solution and in the vapor over the acid are extremely low.

So, the question is &quot;since water vapor is always blown away or reacting in some way and there is only a trace amount of stationery and unreacted water vapor available, why/how is it able to create chemical compounds like phosphoric acid and sulfuric acid?&quot;

Background information
A 1986 paper1 concluded that P4O6 is the main phosphorus bearing gas on Venus from the Vega mission results. It was also reported that some phosphorus bearing particles were present which was found to be phosphoric acid, H3PO4.
I am pretty sure phosphoric acid/phosphorous acid is the result of reaction between phosphorous anhydride and water vapor in sulfuric acid environment (which is later speculated to be the pathway of phosphine formation). But &quot;is water vapor present on Venus sufficient enough to form phosphoric acid? What is the concentration of phosphoric acid on Venusian atmosphere (which would later contribute to formation of phosphine)?&quot;
Reference

Krasnopol'skii, V A. Vega Mission results and chemical composition of Venusian clouds. United States: N. p., 1989. Web. doi:10.1016/0019-1035(89)90168-1.
https://www.vanderbilt.edu/AnS/physics/astrocourses/AST101/readings/water_on_venus.html
Think about the water constantly being both added to and removed from the atmosphere.
Adding water:

Water migrating from deeper layers of the planet crust and mantle into the atmosphere.
Water being formed from sun wind hydrogen and the available oxygen in the atmosphere (the high D/H ratio hints that the primordial water dominates on Venus)
Water being formed from hydrogen and oxygen produced by decomposition of other substances in the atmosphere.

Removing water:

Photolysis - water decomposed to hydrogen and oxygen, both of them engaged in other chemical processes
Escaping hydrogen and possibly whole water molecules into the space.

There is always some equilibrium between the available water and hydrogen and also other hydrogen containing substances like sulphuric acid (that's what Venus clouds are made of) and the widely-discussed today phosphine.

On Earth, we have stratospheric and mesospheric clouds made of water, sulphuric acid and nitric acid in atmospheric layers that are otherwise notoriously dry.  On Venus, the atmosphere has little free oxygen so it is tolerant to much more of a chemical diversity. (E.g. phosphine would self-ignite in Earth's oxygen-rich atmosphere)