I agree that cool rare things should be made from rare in-game materials. I don't agree that this implicitly excludes loot boxes from having their own rare things.
I'm not advocating RNG - ONLY items in any way. I think that's a terrible idea. But at the same time, I want to try and be as realistic as I can about keeping boundless funded. The best way forward for that is to ensue loot boxes are 'worth' buying. Of course, 'worth' is a subjective term here, but it could mean something 'That is worth a lot of in-game-currency to other people'. Or it could be something that you really like. Or things you can't get from any other means.
My reasoning is that a thing is worth more to a single player (beats me why, but psychology is what it is) if it is something that is coveted by others. At the same time, if they feel like it is beyond their means to acquire, they lose interest in having it at all. I think the best way to achieve a balance in that is to provide loot boxes with the potential to have unique things people want in them, and then a back-up that could spend a little more and get the specific thing you want.