You are focusing on one facet that will have to be addressed regardless to change to precursors. You are correct on that, but it by no means invalidates the suggestion.
No matter what, there should to be a change to a broken system. If the dev’s do in fact add any change to precursors as they said they would about a year agoish (Don’t remember the exact date) there will be the same exact problem that any method of change would occur. So, the dev’s would have to consider the other affect on the crafting materials of a legendary.
This method would be a much better alternative to anything suggested so far. This is to include flat out crafting a precursor. Unless, the crafting requires a very very hard to get item that will require RNG to gather.
This suggested method would take out the RNG problem to a large extent. Fulfilling their promise to their consumer base, that there would be a new method to acquiring a precursor while not alienating the base that already has their legendary.
So in recap, absolutely there will be a problem in t6. Will it destroy the market. Not at all. Is the demand going to increase, yes momentarily.
A nice addition that would solve the long term demand increase is to allow only one precursor to be purchased via this method. This would appease those seeking their legendary by giving them a (I can’t stress this enough) far off goal, while not handing them out like candy. This is the best TP stable method I have seen.