now that I think of it these days we have only four real non-pvm money making methods ingame.
which are fishing, farming, herblore, and vials of water. and that's it
let's do some analysis here.
herblore is a skill used to make potions, potions are used in pvm&pvp. which are the two most popular things ingame , and you need vials of water to do that.
so it becomes this way:-
vials of water+herbs>potions>pvmers,pkers buy potions>skillers buy more resources>loop.
which isn't so bad in my opinion, but is that good enough is the question?
in runescape3 and OSRS, they have tons and tons of money making methods that support pvming/pking, which gives the feeling of variety to the game and wide content. if I were to give a solution it would be making a new boss, add loads of requirements, both high and low level items and skills, so both low and high level players gain from that.
actually that's all I could think of

There's a lot more, main ones being:

• fishing mantas (also small amount of caskets and snape grass)
• cooking mantas
• mining coal
• smithing rune bars (superheating advised)
• woodcutting magics (and small amount of c nests)
• farming herbs
• thieving (herb seeds)

You can make money via other ways as well, but they may not be as fast:

• runecrafting (runes don't sell that well since we have Rod and Z staff + rune saving staves)
• herblore (potions don't sell that well since we don't have many PvMers currently)
• fletching + magic (bolts: they don't sell that well anymore, you may have to wait a long time to sell all of them)
• hunter + magic (alching black salamanders gives you a ton of cash, not the fastest method though)
• smithing (dart tips, arrow tips, unf bolts - sometimes bought for high prices to complete daily tasks)
• Crafting bow strings/collecting flax (this has ups and downs, so it's not the most stable way to make cash)

There are plenty of ways to make money, all of which are not listed above. So it's not as bad as you might think.
I do agree with the point that materials should have a stable market, but unfortunately they don't for various reasons (low activity, many ironmen, many high-level accounts etc). I do not agree with you on adding many requirements. If they were to be added, they should make sense

There's a lot more, main ones being:

• fishing mantas (also small amount of caskets and snape grass)
• cooking mantas
• mining coal
• smithing rune bars (superheating advised)
• woodcutting magics (and small amount of c nests)
• farming herbs
• thieving (herb seeds)

You can make money via other ways as well, but they may not be as fast:

• runecrafting (runes don't sell that well since we have Rod and Z staff + rune saving staves)
• herblore (potions don't sell that well since we don't have many PvMers currently)
• fletching + magic (bolts: they don't sell that well anymore, you may have to wait a long time to sell all of them)
• hunter + magic (alching black salamanders gives you a ton of cash, not the fastest method though)
• smithing (dart tips, arrow tips, unf bolts - sometimes bought for high prices to complete daily tasks)
• Crafting bow strings/collecting flax (this has ups and downs, so it's not the most stable way to make cash)

There are plenty of ways to make money, all of which are not listed above. So it's not as bad as you might think.
I do agree with the point that materials should have a stable market, but unfortunately they don't for various reasons (low activity, many ironmen, many high-level accounts etc). I do not agree with you on adding many requirements. If they were to be added, they should make sense

I agree with your disagreements, actually the reason why I said add A boss, was because that's all I could come up with, I wouldn't suggest many requirements that don't make any sense, but merely in a non-direct way asking for suggestions that could balance both low and high requirements.
.e.g
that boss drops a certain weapon, or shield, and that shield needs stuff you get from a various of skills