Someone one does worry how long consumers will be allowed to get their hands on STPP. I mean what is the point of removing a substance from consumer products, if those same persons go out and add it right back.
Regarding the query regarding storage of STPP, am currently using vintage boxes of "Calgon" which is made with two types of phosphates, and it still flows freely and to my tests/knowledge (pH testing,etc) it has not broken down into TSP. Boxes are silver in colour.
Suppose one could order STPP in huge sealed poly sacks, or store in air tight containers, and it should last quite awhile. Air, moisture and IIRC, heat will degrade the product, so need to be avoided.
Only problem is that one really requires so little STPP by volume in ratio to detergent, that it could take ages to go through anything >25lbs.