Scape games, regardless of RSPS or not, were always grind based which means for anything you do grind certain amount of time to achieve something(staking excluded, since if you win all the time, you make bank easily). What destroys economy in my eyes is plain staker who buys items for average or high price, gets bank high in cash and sells those items below average price which makes up for merchers huge profit and leaves other money making methods to be considered poor, according to gp/h wise.
Off-topic: Closing Duel Arena won't solve this issue but tax in it for staking items would probably do the trick. Not sure who but it was suggested before it can be planned in future as well for economy purposes.
As for pure money making methods. Boy, where should I begin with... Right before double exp weekend came out - bow strings, magic logs, dragonhides, raw mantas, metal bars and probably, rune ess had such high demand I don't know where to start... To get by economy just watch what's in demand, has stable price (around 5k gp difference or so per item) and start gathering. Players are becoming aware of expensive items and doesn't want to gather things nowadays, majority of player is skipping what made them money in first place. For example, take Rune ess. You can make up to 8M/h consistently if doing it right and with certain items which you can find in wiki. Another one is vial of water. Suggestion just came out to adding it to Aquanite drop table but I bet it won't change price of it a lot if quantity will be around 10 - 20 noted per drop. So, buying from general store costs 1gp, filling at someone's house where bank and sink is close to each other. with this process it is also possible to make 8M/h. Not sure about other beginner methods but flax picking isn't bad either, probably 10M/h can be made (not sure of profit, since prices varies). But these are only beginner methods where players at 3 level can start(by 3 level I mean, right after the tutorial island finish) Then again, it depends on players who buy them as well. You just gotta keep watching prices and stay tuned. Gotta have your favourite method of making money.
I am not best at proving facts but you CAN get around this economy as long as it has player base in it that actively uses marketing.