As I see it, there are two problems with what you have stated.
The first is that the current system does not stop people from getting the available missions they want, it simply makes doing so frustrating and annoying. Making things frustrating and annoying simply turns players off and is not good game design.
Better design would be to allow players to directly select any possible available mission in a given tier, and balance the supply through the number of concurrent missions available and the mission returns.
The second goes back to balancing mat supply with mat demand. Even if we assume the probability of any given mission appearing in the list is the same as any other mission, we still are left with the fact that metal has a significantly higher demand than cloth, yet does not have a higher number of possible missions.
Solutions here include increasing the number of metals missions so the ratio of metals to cloth matches the demand / usefulness to simply removing cloth and having all crew skills use metals to chainging all force crafted items to use cloth and all non-force crafted items to use metal.
Between Cybertech, ArmorMECH (not armorTECH), and Synthweaving, roughly 89% of their combined schematics require Underworld Metals. Perhaps someone from BioWare would care to explain
why, if that is the case, only 25% of Underworld Trading missions are Underworld Metals? No?
Well, I know a good word for it: Sadism.