Forbidden materia is a simple supply/demand situation. If the Tier 4 supply greatly exceeds the demands of people 1-slotting gear, then prices will probably/hopefully drop enough to enable gambling on 2 and 3-slotting it. I don't think anyone would go spend several mil on a materia to have a 7% chance of adding it. In the meantime I think most people will just multiple up on low tier materia and wait it out.

I agree that it's a bit sad that the entire materia system is just a long grind against a random number generator. But since right now DoL/DoH and spiritbond farming are pretty much 0-skill activities, this was the easiest way to place limits on it. Hopefully they improve on the system, because there's a lot of room to make it better and more enjoyable.