I agree mostly this would be ideal. The case I'm keen to aim for is that there is a meaningful monetary progression ladder for mains...
E.G. Slash Weapons...
- Cool Sword - 100k
- Really Cool Sword - 15m
- Uber Cool Sword - 100m
Realistically they'd have requirements too, say 70, 75, 80.
It's important that the Really Cool Sword doesn't end up so worthless its also close to 100k. It just removes the need for Cool Sword as an option except for players between 70-75 which is obviously done relatively easily.
All of this is to say is we do see monetary progression as a means to have different tiers of power too - which means we do have to care about the price within reason and its very easy for a non BIS weapon, like Really Cool Sword is to be oversupplied, given the Uber Cool Sword isn't that out of reach.
Hence the kind of contradiction I've spoken about a bit - 'niche' stuff ends up wanting to be quite rare if it wants to sit in a nice price progression tier. We don't necessarily have all the answers, but we're aiming for a sweet spot which can be hard to get right.
Ultimately it is on us to design things in such a way that the player driven economy would make this the case.
I think its actually a lot easier to get it right if the regular loot is poor, i.e. the regular loot is not really an incentive to participate in an encounter. So in the example of tormented demons, your main incentive is merely the price of the claws/synapse. If the price is too low as a main, you choose to do something else - thus lowering supply until it reaches some sort of equilibrium with demand.
Lots for us to continue thinking about but really this is all to say these are the sort of things we are considering and we do care about getting it right. It really is not true that we're trying to balance things around streamers doing 15h sessions and artificially extend life of content.